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A1 noun #1,500 más común 9 min de lectura

Segunda-feira

Top 1,500

At the A1 level, you should focus on the basic identification of 'segunda-feira' as the first day of the work week. You need to know how to say 'Today is Monday' (Hoje é segunda-feira) and how to use the preposition 'na' to say 'on Monday'. At this stage, don't worry too much about the history or complex grammar; just treat it as a vocabulary word for scheduling. You should be able to recognize it in a calendar and understand it when someone tells you a day for a meeting. It is one of the first seven 'time' words you will learn. Practice saying the days in order: segunda, terça, quarta, quinta, sexta, sábado, domingo. Remember that 'segunda' is feminine, so use 'a' or 'na'.

At the A2 level, you begin to use 'segunda-feira' in more complex sentences involving routines and past/future events. You should be able to say 'I go to the gym on Mondays' (Eu vou ao ginásio às segundas-feiras) using the plural form correctly. You will also learn to use 'próxima' and 'passada' to navigate time. At this level, you should also be comfortable dropping the '-feira' in casual conversation, as this is how most native speakers talk. You might start to notice the word in simple advertisements or store hours. Understanding that 'de segunda a sexta' means the work week is a key milestone for A2 learners.

At the B1 level, you can discuss your feelings about 'segunda-feira' and use it in professional contexts. You should be able to explain your weekly schedule in detail, using 'segunda-feira' as a reference point. You will start to encounter common idioms or cultural attitudes toward Mondays, such as the idea of 'recomeço' (starting over). You should be able to write a basic email or text message arranging an appointment on a Monday without making gender or preposition errors. Your pronunciation should be clear, especially the nasal 'en' in 'segunda' and the 'ei' diphthong in 'feira'.

At the B2 level, you should have a firm grasp of the nuances of 'segunda-feira'. You can participate in office small talk about how 'heavy' or 'slow' a Monday feels. You understand the difference between formal writing (using the hyphen and full word) and informal digital communication. You can use 'segunda-feira' in conditional sentences, such as 'Se não fosse segunda-feira, eu sairia para jantar' (If it weren't Monday, I would go out for dinner). You are also aware of the cultural significance of Monday in different Lusophone countries, such as the 'segunda-feira gorda' in Brazilian Carnival.

At the C1 level, you understand the deep historical and liturgical roots of why Portuguese uses 'segunda-feira' instead of a name based on the Moon. You can discuss the linguistic evolution from Latin 'secunda feria' and the influence of Saint Martin of Braga. You use the word with total native-like fluency, including all contractions and plural forms. You can appreciate literature or complex news articles where 'segunda-feira' might be used metaphorically to represent bureaucracy, routine, or the mundane. Your use of the word is indistinguishable from a native speaker, including the subtle shortening to 'segunda' in the appropriate registers.

At the C2 level, you have mastered the sociolinguistic variations of 'segunda-feira' across the entire Lusophone world. You can analyze the word's role in the unique identity of the Portuguese language compared to other Romance languages. You are comfortable using the word in highly academic or legal contexts, understanding its implications in 'prazos' (deadlines) and 'dias úteis'. You can even use or understand rare or slang forms like 'segundou' or 'segundona' with perfect timing and irony. You are capable of debating the merits of the liturgical naming system versus the planetary one, using 'segunda-feira' as a prime example of Portuguese linguistic exceptionalism.

Practícalo Ejemplos Notas y consejos

Segunda-feira en 30 segundos

Segunda-feira is the Portuguese word for Monday, the first business day of the week.
It is a feminine noun, always used with feminine articles like 'a' or 'na'.
Unlike Spanish or French, Portuguese uses a numerical system (2nd fair) for weekdays.
In casual speech, it is often shortened to just 'segunda', but the full form is formal.

The term Segunda-feira is the Portuguese word for Monday. Unlike most Romance languages that derive their names for the days of the week from celestial bodies (like the Moon for Monday/Lunes/Lundi), Portuguese employs a unique liturgical numbering system. This system dates back to the 6th century, specifically the year 563 AD, following the Council of Braga. Saint Martin of Braga argued that Christians should not use the names of pagan gods (Mars, Venus, etc.) to name the days of the week. Consequently, Portuguese adopted a system where the days are 'fairs' or 'festivals' (feiras), starting with Sunday as the 'First Day' (Domingo) and Monday as the 'Second Fair' (Segunda-feira).

Etymological Origin: Derived from the Latin 'secunda feria', meaning the second day of the week in the ecclesiastical calendar.

A segunda-feira é o primeiro dia útil da semana para a maioria das pessoas.

In a broader sense, Segunda-feira represents the commencement of the professional and academic cycle. It carries a heavy cultural weight, often associated with the transition from leisure to responsibility. While technically the second day of the week (Sunday being the first), it is functionally the 'primeiro dia útil' (first business day). This duality is crucial for learners to understand: the name says 'second', but the lifestyle says 'start'.

Grammatical Gender: It is a feminine noun. Therefore, you use 'a segunda-feira' and 'na segunda-feira' (em + a).

Detesto acordar cedo na segunda-feira.

Culturally, the word is often shortened to just 'segunda' in casual conversation. For example, 'Vemo-nos na segunda' (See you on Monday). The suffix '-feira' is almost always dropped in informal speech, though it remains mandatory in formal writing, news broadcasts, and official documentation. This linguistic flexibility is a hallmark of European and Brazilian Portuguese alike.

A reunião foi adiada para a próxima segunda-feira.

Plural Form: The plural is 'segundas-feiras'. Both parts of the compound word are pluralized because both are nouns/adjectives.

Às segundas-feiras, o museu está fechado para manutenção.

Que segunda-feira preguiçosa!

To reach the depth required for a full understanding, one must also consider the psychological impact of the word. In Lusophone cultures, 'segunda-feira' is the day of 'recomeço' (new beginning). It is the day people start diets, new projects, or return to the gym. It is the antithesis of 'domingo' (Sunday), which is traditionally a day for family and rest. The transition from the 'primeira-feira' (which became Domingo) to the 'segunda-feira' marks the shift from the sacred/restful to the secular/productive.

Using Segunda-feira correctly involves mastering prepositions and context. The most common preposition used with days of the week is 'em', which contracts with the feminine article 'a' to become 'na'. If you want to say 'on Monday', you say 'na segunda-feira'. If you are referring to Mondays in general (habitual actions), you use 'às segundas-feiras' (at the Mondays).

Prepositional Usage: Specific: Na segunda-feira (On Monday). Habitual: Às segundas-feiras (On Mondays).

Vou ao médico na segunda-feira de manhã.

When discussing schedules, you might use 'de' and 'a' to indicate a range. For example, 'de segunda a sexta' (from Monday to Friday). Notice that in this construction, we often drop the '-feira' and the articles for brevity. This is standard in business hours and academic calendars. It sounds more natural and professional.

Formal vs. Informal: Formal: Segunda-feira. Informal: Segunda.

A loja abre de segunda a sábado.

Another important aspect is the use of 'próxima' (next) and 'passada' (last). 'Na próxima segunda-feira' refers to the upcoming Monday, while 'na segunda-feira passada' refers to the one that just occurred. In some regions, especially in Brazil, people might say 'segunda que vem' to mean next Monday. This is very common in spoken language.

Eu te ligo na segunda que vem.

Time of Day: Segunda-feira de manhã (Monday morning), segunda à tarde (Monday afternoon), segunda à noite (Monday night).

Temos um compromisso na segunda à noite.

In writing, the hyphen is essential. The New Orthographic Agreement (Acordo Ortográfico) maintained the hyphen for days of the week. Failing to use it in a formal essay or business email is a common spelling error. However, when you drop the '-feira', the hyphen also disappears (e.g., 'na segunda').

Não gosto de compromissos logo na segunda-feira cedo.

Finally, consider the verb 'segundar'. While not an official dictionary entry in the sense of 'to Monday', it is used in Brazilian internet slang to describe the act of starting the week or feeling the typical Monday vibes. 'Segundou!' is a popular social media caption used on Monday mornings to acknowledge the start of the work week, similar to 'sextou' for Friday.

You will encounter Segunda-feira in almost every facet of daily life in Portuguese-speaking countries. From the moment you turn on the news to a casual chat at a café, the word is ubiquitous. In professional settings, it is the anchor for the work week. Project deadlines, weekly meetings, and 'relatórios' (reports) are almost always tied to the 'segunda-feira'.

In the Office: 'A reunião de equipe é toda segunda-feira às nove.'

Preciso deste documento pronto na segunda-feira.

In the media, weather forecasts and news anchors use the full term 'segunda-feira' to maintain a formal register. You'll hear it in phrases like 'A previsão para segunda-feira indica chuva' (The forecast for Monday indicates rain). It is also the day when many cultural institutions, like museums and some restaurants, are closed, so you will often see signs saying 'Fechado às segundas-feiras'.

In Schools: Students use it to refer to the start of classes or the day homework is due.

As aulas começam na segunda-feira.

In popular music and literature, Monday is often depicted as a day of melancholy or transition. Brazilian Samba and Bossa Nova songs occasionally mention 'segunda-feira' as the day the party ends and reality sets in. It is the day the 'fantasia' (costume) of Carnival is put away. Hearing 'segunda-feira' in a song often sets a tone of sobriety or longing for the weekend.

A segunda-feira chegou e o carnaval acabou.

Public Services: Banks and government offices typically have their longest queues on Monday mornings.

O banco está sempre cheio na segunda-feira.

In the digital world, 'Segunda-feira' is a trending topic every week. Memes about 'café' (coffee) and 'sono' (sleepiness) dominate social media feeds in Brazil and Portugal. If you are following Portuguese-speaking influencers, you will see 'Bom dia, segunda-feira!' as a common greeting to start the week with positive energy, often accompanied by a photo of a desk or a coffee cup.

One of the most frequent errors for English speakers is forgetting that Segunda-feira is feminine. In English, days of the week don't have gender, but in Portuguese, 'feira' is a feminine noun, making the entire day feminine. This affects the articles and adjectives used with it. You must say 'A segunda-feira' and 'Uma segunda-feira cansativa'.

Gender Error: Incorrect: O segunda-feira. Correct: A segunda-feira.

Esta segunda-feira está sendo muito produtiva.

Another common pitfall is the preposition. English speakers often want to say 'em segunda-feira' (in Monday) or 'no segunda-feira' (on the Monday - masculine). The correct form is 'na segunda-feira'. The contraction 'na' (em + a) is mandatory when referring to a specific Monday. If you forget the article and just say 'em segunda-feira', it sounds very unnatural and broken.

Hyphenation: Always use a hyphen in 'segunda-feira'. It is a compound noun.

Escrevi segunda-feira sem hífen e o professor corrigiu.

Pluralization is also tricky. Some learners try to pluralize only the second part ('segunda-feiras') or only the first part ('segundas-feira'). The correct way is to pluralize both: 'segundas-feiras'. This is because 'segunda' acts as an adjective (second) and 'feira' as a noun, and in Portuguese compound words of this type, both elements agree in number.

Eu trabalho de casa às segundas-feiras.

Capitalization: In Portuguese, days of the week are NOT capitalized unless they start a sentence.

Hoje é segunda-feira (not Segunda-feira).

Lastly, don't confuse 'segunda-feira' with 'segundo' (second - the unit of time or the ordinal number). While they share the same root, they are used differently. You wouldn't say 'um segunda-feira' to mean 'one second'. Similarly, 'segunda' can mean 'second' (feminine ordinal), so context is key. 'A segunda vez' (the second time) vs 'A segunda-feira' (Monday).

While Segunda-feira is the standard term, there are several related words and synonyms depending on the context. The most common 'synonym' is the shortened version, segunda. This is used in 90% of spoken conversations. It is less formal but perfectly acceptable in all but the most rigid professional environments.

Segunda vs. Segunda-feira: 'Segunda' is the informal, spoken shorthand. 'Segunda-feira' is the full, formal name.

A gente se vê na segunda, então?

Another related term is dia útil (business day/work day). Since Monday is the first business day of the week, it is often referred to as the 'primeiro dia útil'. In legal contracts or bank statements, you might see this instead of the specific day name. It emphasizes the function of the day rather than its position in the week.

Início de semana: Means 'start of the week'. Often used interchangeably with Monday in general contexts.

Desejo a todos um ótimo início de semana.

In Brazil, you might hear the term ressaca de segunda (Monday hangover). This doesn't necessarily mean a literal hangover from alcohol, but rather the sluggishness and difficulty of getting back into the work routine after the weekend. It is a common cultural expression used to justify a slow start to the day.

Estou com aquela ressaca de segunda hoje.

Segunda-feira gorda: Refers to the Monday of Carnival, a day of intense celebration before Ash Wednesday.

A segunda-feira gorda é o auge do desfile.

Finally, compare 'segunda-feira' with its counterparts: 'terça-feira' (Tuesday), 'quarta-feira' (Wednesday), etc. Understanding the pattern (Segunda, Terça, Quarta, Quinta, Sexta) is essential. Unlike English, where the names are distinct, the Portuguese system is numerical and rhythmic, making it easier to memorize once you know the numbers 2 through 6.

How Formal Is It?

Formal

Informal

Jerga

Nivel de dificultad

Gramática que debes saber

Ejemplos por nivel

Hoje é segunda-feira.

Today is Monday.

Use 'é' for 'is'.

Eu trabalho na segunda-feira.

I work on Monday.

Na = em + a.

A aula é segunda-feira.

The class is Monday.

No preposition needed sometimes in simple statements.

Segunda-feira é um dia longo.

Monday is a long day.

Feminine adjective 'longa' if referring to 'a jornada'.

Até segunda-feira!

See you Monday!

Common farewell.

Ela estuda na segunda.

She studies on Monday.

Shortened 'segunda' is common.

O mercado abre na segunda-feira.

The market opens on Monday.

Standard sentence structure.

Não gosto de segunda-feira.

I don't like Monday.

Negative 'não' before the verb.

Eu vou ao ginásio às segundas-feiras.

I go to the gym on Mondays.

Plural 'às segundas-feiras' for habits.

Na segunda-feira passada, eu fui ao cinema.

Last Monday, I went to the cinema.

Past tense 'fui'.

O que você vai fazer na próxima segunda?

What are you going to do next Monday?

Future 'vai fazer'.

A loja fecha na segunda-feira à tarde.

The shop closes on Monday afternoon.

Time of day 'à tarde'.

Preciso comprar pão na segunda.

I need to buy bread on Monday.

Infinitive 'comprar'.

Minha consulta é na segunda-feira de manhã.

My appointment is on Monday morning.

Possessive 'minha'.

Eles viajam toda segunda-feira.

They travel every Monday.

'Toda' implies every.

A reunião começa na segunda às oito.

The meeting starts on Monday at eight.

Preposition 'às' for time.

Sempre me sinto cansado na segunda-feira de manhã.

I always feel tired on Monday morning.

Reflexive verb 'sentir-se'.

Se a segunda-feira for feriado, vamos viajar.

If Monday is a holiday, we will travel.

Future subjunctive 'for'.

O relatório deve ser entregue na segunda-feira impreterivelmente.

The report must be delivered on Monday without fail.

Passive voice 'ser entregue'.

Costumo organizar a minha agenda na segunda-feira cedo.

I usually organize my schedule early on Monday.

Verb 'costumar'.

A segunda-feira é o dia internacional de começar a dieta.

Monday is the international day to start a diet.

Cultural idiom.

Não pude ir à festa porque tive que trabalhar na segunda.

I couldn't go to the party because I had to work on Monday.

Compound past 'tive que'.

O trânsito na segunda-feira é sempre terrível.

Traffic on Monday is always terrible.

Noun 'trânsito'.

Espero que a segunda-feira passe rápido.

I hope Monday passes quickly.

Present subjunctive 'passe'.

A letargia da segunda-feira desaparece após o primeiro café.

Monday's lethargy disappears after the first coffee.

Advanced vocabulary 'letargia'.

Muitas pessoas sofrem da 'síndrome de segunda-feira'.

Many people suffer from 'Monday syndrome'.

Idiomatic expression.

O museu permanece encerrado às segundas-feiras para manutenção.

The museum remains closed on Mondays for maintenance.

Formal verb 'permanecer'.

A segunda-feira de Carnaval é um dos dias mais animados no Brasil.

Carnival Monday is one of the most lively days in Brazil.

Superlative 'mais animados'.

Caso a reunião seja na segunda, avisaremos com antecedência.

In case the meeting is on Monday, we will notify you in advance.

Conjunction 'caso' + subjunctive.

A produtividade tende a ser menor na segunda-feira de manhã.

Productivity tends to be lower on Monday morning.

Verb 'tender a'.

Ela detesta a monotonia das segundas-feiras no escritório.

She hates the monotony of Mondays in the office.

Abstract noun 'monotonia'.

A segunda-feira marca o reinício do ciclo produtivo.

Monday marks the restart of the productive cycle.

Formal register.

A nomenclatura 'segunda-feira' reflete a herança cristã de Portugal.

The nomenclature 'Monday' reflects Portugal's Christian heritage.

Academic vocabulary.

O termo deriva do latim 'secunda feria', instituído por São Martinho de Braga.

The term derives from the Latin 'secunda feria', established by Saint Martin of Braga.

Historical reference.

A segunda-feira é frequentemente imbuída de um sentimento de renovação.

Monday is often imbued with a sense of renewal.

Passive participle 'imbuída'.

Nas entrelinhas da segunda-feira, reside a promessa de uma semana profícua.

Between the lines of Monday lies the promise of a fruitful week.

Literary style.

A aversão à segunda-feira é um fenómeno sociológico globalizado.

Aversion to Monday is a globalized sociological phenomenon.

Complex noun phrase.

É imperativo que o cronograma seja cumprido a partir de segunda-feira.

It is imperative that the schedule be met starting Monday.

Impersonal expression + subjunctive.

A segunda-feira, por ser o primeiro dia útil, concentra a maior carga burocrática.

Monday, being the first business day, concentrates the greatest bureaucratic load.

Causal clause.

Subjacente ao conceito de segunda-feira está a dicotomia entre lazer e labor.

Underlying the concept of Monday is the dichotomy between leisure and labor.

Inverted sentence structure.

A singularidade do sistema de 'feiras' em Portugal é um testemunho da resistência cultural.

The uniqueness of the 'fairs' system in Portugal is a testament to cultural resistance.

High-level abstract concepts.

A segunda-feira atua como o limiar psicológico entre o sagrado e o profano.

Monday acts as the psychological threshold between the sacred and the profane.

Philosophical terminology.

Não obstante o desdém popular, a segunda-feira é o esteio da organização social.

Notwithstanding popular disdain, Monday is the mainstay of social organization.

Concessive conjunction 'não obstante'.

A semântica de 'segunda-feira' transcende a mera cronologia semanal.

The semantics of 'Monday' transcends mere weekly chronology.

Linguistic analysis.

A segunda-feira é o palco onde se desenrola a dialética do recomeço.

Monday is the stage where the dialectic of starting over unfolds.

Metaphorical usage.

Eis que a segunda-feira desponta, trazendo consigo o peso da responsabilidade civil.

Behold, Monday dawns, bringing with it the weight of civil responsibility.

Archaic/Literary 'Eis que'.

A percepção da segunda-feira é moldada por constructos sociais de produtividade.

The perception of Monday is shaped by social constructs of productivity.

Sociological vocabulary.

A segunda-feira, em sua essência, é a negação da efemeridade do fim de semana.

Monday, in its essence, is the negation of the weekend's ephemerality.

Complex philosophical structure.

Colocaciones comunes

Próxima segunda-feira

Segunda-feira passada

Toda segunda-feira

Na segunda-feira de manhã

Segunda-feira à tarde

Segunda-feira à noite

Segunda-feira feriado

Manhã de segunda-feira

Rotina de segunda-feira

Trânsito de segunda-feira

Frases Comunes

Bom início de semana!

Até segunda!

Na segunda a gente conversa.

Segunda-feira é dia de branco.

Odeio segunda-feira.

Segunda-feira sem falta.

De segunda a sexta.

Segunda-feira brava.

Segunda-feira de manhã cedo.

Logo na segunda-feira?

Se confunde a menudo con

Segunda-feira vs Segundo

Unit of time (second) or masculine ordinal number.

Segunda-feira vs Segunda

Feminine ordinal number (e.g., second time).

Segunda-feira vs Feria

Latin root, but 'férias' in Portuguese means vacation.

Modismos y expresiones

Fácil de confundir

Segunda-feira vs

Patrones de oraciones

Cómo usarlo

note

While 'segunda' means 'second', in the context of days, it always implies Monday. In a race, 'chegar em segundo' means to arrive in second place, but 'chegar na segunda' means to arrive on Monday.

Errores comunes

O segunda-feira
No segunda-feira
Segunda feira (without hyphen)
Segunda-feiras (plural)
Segunda-Feira (capitalized)

Consejos

Gender Agreement

Always remember that 'segunda-feira' is feminine. If you describe your Monday, use feminine adjectives. For example, 'uma segunda-feira produtiva' (a productive Monday). This is a very common mistake for beginners who are used to genderless days in English.

Be Natural

To sound like a native, drop the '-feira' in 90% of your conversations. If a friend asks when you're free, say 'Na segunda' instead of 'Na segunda-feira'. It sounds much more natural and less like a textbook.

The 'Feira' System

Portuguese is unique among Romance languages for this system. While Spanish uses 'Lunes' (Moon), Portuguese uses 'Segunda-feira'. This is a great conversation starter with natives about the history of the language and the influence of the Church.

Hyphen Rules

In formal writing, the hyphen is your friend. The 1990 Orthographic Agreement kept hyphens for days of the week. 'Segunda-feira', 'terça-feira', etc., all need them. Skipping them is a sign of poor spelling in a professional context.

Nasal Vowels

Pay close attention to the 'un' in 'segunda'. It's a nasal vowel, meaning the air goes through your nose. Don't pronounce a hard 'n' like in the English word 'sun'. It's closer to the 'un' in the French word 'un'.

Work Week

Learn the phrase 'dia útil'. Monday is the 'primeiro dia útil'. If you see a sign that says 'Aberto nos dias úteis', it means they are open Monday through Friday. This is vital for navigating cities.

Segundou

In Brazil, you'll see 'Segundou!' on social media. It's a play on 'Sextou!' (Friday arrived!). It's used to acknowledge that the work week has begun, often with a mix of motivation and irony.

Numerical Order

If you know your numbers (dois, três, quatro, cinco, seis), you know the days. Segunda (2nd), Terça (3rd), Quarta (4th), Quinta (5th), Sexta (6th). Monday is the 2nd day of the week.

The 'R' in Feira

The 'r' in 'feira' is a 'flap r'. It's made by tapping the tip of your tongue against the roof of your mouth once. It's not the guttural 'r' from the back of the throat used at the start of words.

Monday Blues

In Lusophone cultures, complaining about Monday is a standard social bonding activity. If you want to start a conversation at work, a simple 'Que segunda-feira difícil, hein?' (Difficult Monday, huh?) will always get a response.

Memorízalo

Mnemotecnia

Monday is the SECOND (Segunda) day of the week, and you go to the FAIR (Feira) to start work.

Origen de la palabra

Latin 'secunda feria'

Contexto cultural

Commonly shortened to 'segunda'. 'Segundou' is a popular social media hashtag.

More likely to use the full 'segunda-feira' in formal settings. 'Segunda' is also the name for the 2nd grade in school.

Practica en la vida real

Contextos reales

Inicios de conversación

"Como foi sua segunda-feira?"

"Você trabalha na segunda-feira?"

"O que você costuma fazer às segundas-feiras?"

"Você também acha a segunda-feira o dia mais difícil?"

"Vamos marcar um café para a próxima segunda?"

Temas para diario

Descreva sua rotina típica de segunda-feira.

Por que as pessoas geralmente não gostam de segundas-feiras?

O que você fez na segunda-feira passada?

Quais são seus objetivos para a próxima segunda-feira?

Escreva sobre um feriado que caiu em uma segunda-feira.

Preguntas frecuentes

10 preguntas

It comes from the Latin 'secunda feria'. The Catholic Church in Portugal renamed the days to avoid pagan names like the Moon or Mars. Sunday is the first day, so Monday is the second.

No, in casual speech, most people just say 'segunda'. However, in formal writing, news, or official documents, you should use the full form 'segunda-feira'.

It is feminine. You say 'a segunda-feira' and use feminine adjectives like 'cansativa' or 'longa'. This is because 'feira' is a feminine noun.

You say 'às segundas-feiras'. The 'às' is a contraction of 'a' (preposition) and 'as' (feminine plural article), indicating a habitual action.

Yes, grammatically the hyphen is required in 'segunda-feira'. If you drop the '-feira' and just say 'segunda', then you don't use a hyphen.

No, unlike English, days of the week are not capitalized in Portuguese unless they are at the beginning of a sentence.

It means 'from Monday to Friday'. It is the standard way to describe a typical work or school week in Portuguese-speaking countries.

It is the Monday of Carnival week. It's a day of big celebrations and parades in Brazil and some parts of Portugal before Lent begins.

The plural is 'segundas-feiras'. Both words are pluralized because 'segunda' is an adjective and 'feira' is a noun in this compound structure.

No, that is a common mistake. Since 'segunda-feira' is feminine, you must use 'na' (em + a). 'No' is only for masculine nouns.