Learn Finnish: Juhla oli ihan mukava. - The party was completely nice.

You attach the question particle -kö to the negative verb ei, giving you eikö. Remember that in the past tense negative, the main verb changes to its past participle form. So oli becomes ollut, making the full question: Eikö juhla ollut ihan mukava?

The translation says "completely nice", but doesn't ihan usually mean "quite"?

Yes, ihan is a very versatile word. It can mean "quite", "pretty", "completely", or "totally". When Finns say ihan mukava, they usually mean "quite nice" in a mildly positive way, though it can mean "completely nice" if emphasized. Context and tone of voice make the difference!

Why isn't the adjective in the partitive case (mukavaa)?

Because juhla is a singular, countable noun. In a basic describing sentence, the adjective matches the subject's form. You would use the partitive only if the subject was an uncountable mass noun or an abstract concept, for example: Musiikki oli mukavaa (The music was nice).

I usually see the word for party in the plural (juhlat). Why is it singular here?

Both juhla (singular) and juhlat (plural) can be used to talk about a single party. The plural is very common for social gatherings, but the singular works perfectly too. If you used the plural, the rest of the sentence would simply have to match it: Juhlat olivat ihan mukavat.