Learn Spanish (Spain): El viernes tenemos una fiesta.

In Spanish, you don't use a preposition like "on" for days of the week. Instead, you just use the masculine word for "the" (el). So, el viernes literally means "the Friday", but it is how you say "on Friday".

Viernes ends in an "s". Does that mean it translates to "on Fridays" plural?

No, viernes is singular here, meaning just one Friday. In Spanish, the days of the week from Monday to Friday naturally end in "s" (lunes, martes, miércoles, jueves, viernes). You can tell it is singular because it uses el instead of los.

How would I say "On Fridays we have a party" if it happens every week?

You would change the article from el to los. Because the word viernes already ends in an "s", the day itself doesn't change spelling. So, "on Fridays" as a routine is simply los viernes.

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

In Spanish, you can usually leave out the pronoun (like nosotros for "we") because the verb ending already tells you who is doing the action. The -emos ending in tenemos specifically means "we have".