Saying 'on' a day of the week
In English, we say "on Monday" or "on Fridays" using the preposition "on" without any article. In Portuguese, expressing "on" a day of the week requires you to combine a preposition with a definite article. It is almost like saying "in the Monday" or "to the Mondays".
English speakers often trip over this because the choice of word depends on two things. First, it depends on whether the day is masculine or feminine. Second, it depends on whether the event is happening just once or is a regular habit.
When you are talking about a specific day, such as an upcoming meeting or a party this weekend, Portuguese uses the preposition em combined with a definite article.
Monday through Friday are feminine words. They all end in -feira. Because they are feminine, the preposition em merges with the feminine article a to become na. You use na for segunda-feira, terça-feira, quarta-feira, quinta-feira, and sexta-feira.
The weekend days, sábado and domingo, are masculine. For these, em merges with the masculine article o to become no.
When you want to describe something you do regularly, like "on Mondays" or "every Sunday", Portuguese often switches to the preposition a combined with a plural definite article.
For the feminine weekdays, a merges with as to form às. This little word includes a grave accent to show that two words have contracted. So, "on Tuesdays" becomes às terças-feiras, and "on Wednesdays" becomes às quartas-feiras.
For the masculine weekend days, a merges with os to become aos. "On Sundays" translates to aos domingos.
In everyday spoken Portuguese, it is very common to drop the -feira part of the weekday names. Instead of saying segunda-feira or quinta-feira, you will frequently hear just segunda or quinta.
The rules for prepositions remain exactly the same. You still use na for a specific day and às for regular habits, because words like terça, quarta, and sexta remain feminine words.
To say "on" a day of the week, you must remember the gender of the day. Weekdays are feminine and take na for a single event or às for a repeating habit. The weekend days are masculine and take no for a single event or aos for a habit.