Learn Italian: Martedì prossimo andiamo alla festa. - Next Tuesday we are going to the party.

Unlike full calendar dates which use il (like il 5 maggio), you don't use an article for a specific upcoming day of the week. If you said il martedì, it would change the meaning to "on Tuesdays" or "every Tuesday".

Why does prossimo come after Martedì instead of before it?

In Italian, adjectives (describing words) usually come right after the noun they describe. So "next Tuesday" is literally "Tuesday next" (Martedì prossimo). The same rule applies to scorso (last), as in giovedì scorso (last Thursday).

Since the party is in the future, why do we use the present tense word andiamo?

Just like in English where we can say "we are going to the party next week," Italians often use the present tense (andiamo) to talk about firmly planned future events. You don't need a special future tense here!

Where does alla come from in alla festa?

You are going "to the" party, so you need the preposition a (to) and the article la (the, because festa is feminine). In Italian, a + la must merge into the single combined word alla.