Days of the week, months, and dates
Two surprises before we start. First: in Portuguese, days of the week and months are lowercase. segunda-feira, janeiro — never with a capital, except at the start of a sentence. Second: weekdays are numbered. Monday is segunda-feira ('second-day'), Tuesday is terça-feira, and so on through Friday. Only Saturday and Sunday have proper names: sábado and domingo.
The seven days
segunda-feira = Monday. terça-feira = Tuesday. quarta-feira = Wednesday. quinta-feira = Thursday. sexta-feira = Friday. sábado = Saturday. domingo = Sunday. In everyday speech the -feira half is almost always dropped: people just say segunda, terça, quarta, quinta, sexta. You'll see the full form on schedules and printed timetables; you'll hear the short form in conversation.
On Monday vs on Mondays
na segunda = on Monday (this coming one). The contraction na = em + a — and yes, segunda-feira is feminine (the noun feira is feminine, and that's what the article agrees with). For 'on Mondays' (every Monday, habitual), use the plural with às: às segundas = on Mondays. Trabalho na segunda = I work on Monday. Trabalho às segundas = I work on Mondays.
The twelve months
janeiro, fevereiro, março, abril, maio, junho, julho, agosto, setembro, outubro, novembro, dezembro. All lowercase. Note março with the ç (s-sound), and agosto without an accent. To say 'in January', use em: em janeiro, em maio. No article needed.
Dates: o 5 de maio
Portuguese dates run day-month-year, glued together with de. o 5 de maio = May 5th. o 25 de abril = April 25th. The o at the front (masculine, agreeing with the unspoken word dia) is normal in writing and speech. For the first of the month, you can say either o 1 de janeiro or o dia 1 de janeiro — both work, and the second form is common when you want to highlight that you mean a specific day. Full dates: 15 de agosto de 2024 = August 15, 2024.
Asking what day / what date
Que dia é hoje? = What day is it today? — answered with the weekday: Hoje é segunda-feira. Que dia é hoje? can also mean 'what's today's date?', answered with the date: Hoje é (dia) 5 de maio. If you want to be unambiguous, Em que dia estamos? ('What date are we on?') always points at the date. Quando é o teu aniversário? = When is your birthday? — answer: No dia 12 de março.
PT vs BR
Brazilian speakers often skip -feira even more aggressively in writing, and tend to say segunda, terça without it ever appearing. Otherwise the system is the same in both registers. Months and weekdays were briefly capitalized in older Brazilian style but the current standard is lowercase in both countries.
