Learn Spanish (Latin America): Los invitados están por festejar y yo acabo de servir la champaña. - The guests are about to celebrate and I just served the champagne.

The phrase estar por followed by an infinitive verb (like festejar) is a special structure that means an action is pending or on the verge of happening. It translates perfectly to 'to be about to' do something.

Could I use para instead of por here and say están para festejar?

In Latin America, estar por plus an infinitive is the standard way to say you are 'about to' do something. Using estar para changes the meaning slightly to being 'in the mood for' or 'ready for' something, rather than being on the verge of actually doing it.

Why does acabo de servir translate as 'I just served' rather than 'I finish serving'?

While the verb acabar usually means 'to finish', the specific formula acabar de plus an infinitive (like servir) means 'to have just done' something. Notice that we use the present tense acabo, even though in English you naturally translate it to the past tense ('I just served').

Why is champagne feminine here (la champaña)? I have seen it as masculine before.

Both are correct! In much of Latin America, the French word is adapted as a feminine noun ending in A: la champaña. In other regions, especially Spain, the masculine adaptation el champán is more common.