There was a party (as in last night). Il y avait une fête hier soir ?? は フランス語 (フランス) で何と言いますか？

質問を翻訳

Whether to use "il y a eu" or "il y avait" depends on the context francly, so let me explain some grammar and then it's up to you to decide the correct one :)

Past tenses in French, just like in English, have a very specific use and even though at first they might seem the same, there is a nuance to it.

The "passé composé" (and it's written variant "passe simple) is used for one-moment actions (and thus sudden actions as well) and a succession of short one-moment actions. You also use this tense to talk about things that happened during a very specific, well-known moment.

The "imparfait" is used for the exact opposite: background actions, descriptions, actions during an undifined period of time and duration.

The best example for this is a fairy tale.

Il y avait un jour une belle princesse qui était très heureuse. Elle vivait dans un beau château et était aimée par tout son peuple.
Un jour il y a eu une catastrophe! La princesse avait tombé dans un sommeil éternelle. Tous les princes sont venus pour la réveiller, mais nulle y est arrivé. Après un long moment pleine d'angoisse, un beau prince, qui avait les cheveux blonds, est venu.

Random story, but I hope that you understand the tenses I used :) in the beginning it's all just background and descriptions, but then there is a change and a one-moment action occurs!

Another example to explain the duration thing:
1. Elle vivait dans cette maison.
2. Elle a vécu dans cette maison pendant 2 ans

In 1, there is no time indication and thus it has all characteristics of a description. 2 however has a specific time indication.