Learn Spanish (Spain): La semana tiene siete días.

The word día is a famous rule-breaker in Spanish! Even though it ends in '-a', which usually means a word is feminine, día is actually masculine. That means we say el día for 'the day' and los días for 'the days'.

Which verb does tiene come from?

It comes from the verb tener, which means 'to have'. Because we are talking about la semana (the week), which counts as a single thing ('it'), we use the matching he/she/it form: tiene.

If I want to say I do something 'on' a day of the week, do I use a specific word for 'on'?

No, and this is a very common trap! To say you do something on a certain day, you simply use the masculine articles el or los before the day. For example, 'on Monday' is el lunes and 'on Mondays' is los lunes.

Does the number siete change its ending if it describes a feminine word?

No, the number siete (seven) stays exactly the same whether it describes masculine or feminine things. Out of the numbers one through ten, only uno (one) changes to match gender, becoming un or una.