Learn Finnish: Menemme gaalaan, upeaan tapahtumaan, katsomaan julkkiksia. - We are going to the gala, a gorgeous event, to watch celebrities.

It is an apposition—a descriptive phrase that renames the noun right next to it (gaalaan). In standard Finnish, an apposition must match the grammatical case of its head noun. Since we are going to the gala (gaalaan is illative), the apposition must also take the illative case.

Do both words in the apposition phrase need to take the illative ending?

Yes. Just like a normal adjective-noun pair, the adjective upea must agree with the noun it modifies (tapahtuma). Because the apposition as a whole must match gaalaan, both words receive the illative ending to become upeaan tapahtumaan.

Why is the verb "to watch" in the form katsomaan rather than the basic dictionary form katsoa?

The main verb here is menemme (we go). In Finnish, when you go somewhere in order to do something, the action verb must be in the third infinitive illative form (the -maan / -mään ending). Even though the apposition is separated by commas, katsomaan is still directly driven by the verb of motion menemme.

Why is julkkiksia in the partitive plural, and how is it formed?