Learn Portuguese (Portugal): Trabalhamos na segunda-feira.

In Portuguese, you almost always use the definite article (the word for 'the') before a day of the week. Because segunda-feira is feminine, you need the feminine 'the', which is a. When you combine the preposition em (in/on/at) with a, it shrinks down into the contraction na. So literally, you are saying 'We work on the Monday'.

Why is Monday called segunda-feira? Does it mean 'second'?

Yes! In Portuguese, the standard weekdays are numbered. Sunday is historically considered the first day of the week, making Monday the second day, or segunda-feira. The word feira originally meant 'market' or 'fair' (dating back to ancient religious naming conventions), but today it just means 'day' when attached to a weekday.

Where is the word for 'we' in this sentence?

The word for 'we' is nós, but Portuguese often drops subject pronouns because the verb ending already tells you exactly who is doing the action. The -amos ending on trabalhamos only ever goes with 'we'. You could say Nós trabalhamos, but leaving it out is very natural and common.

How would I change this sentence to say 'We work on Mondays' (every Monday)?