Learn Portuguese (Portugal): Depois de amanhã é sábado.

The word depois means "after", but when it is followed by a time or event to say "after [something]", it requires the preposition de. So, depois de amanhã literally translates to "after of tomorrow", which is how Portuguese expresses "the day after tomorrow".

Why do we use é (from the verb ser) for a day of the week? Doesn't the day change?

Even though tomorrow will be a different day, Portuguese uses the "permanent" verb ser for dates, times, and days of the week. You are stating a fixed fact about the calendar, so you always use é rather than está.

Why isn't sábado capitalized?

Unlike in English, days of the week and months of the year are not capitalized in Portuguese unless they appear at the very beginning of a sentence.

Should we use an article before sábado, like é o sábado?

No. While Portuguese normally uses definite articles with days of the week (for example, saying no sábado to mean "on Saturday"), you drop the article when you are simply stating what day it is using the verb ser.