Learn Spanish (Spain): Los viernes vamos de tapas. - On Fridays we go out for tapas.

In English, we say "on Fridays", but Spanish just uses the plural definite article (los) before days of the week for things that happen regularly. You never use a word like en for days of the week.

Does the word viernes change form to become plural?

No, days of the week that end in an "s" (Monday through Friday) do not change their spelling in the plural. You only change the article from el to los to show that you mean every Friday rather than just one.

Why do we use de in vamos de tapas instead of a word for "for"?

The phrase ir de is a common pattern in Spanish used for leisure activities where you move from place to place. Just like ir de compras (to go shopping), ir de tapas is a set phrase meaning to go out eating tapas, usually moving between different bars.

Why isn't the word nosotros (we) used before vamos?

The verb vamos already means "we go". Because the "we" is built right into the verb's ending, Spanish speakers usually drop the word nosotros unless they need to clear up confusion or add extra emphasis.