If you're stating a fact about something in the past, then using the past tense (fue) is always correct. In this case the party is over, and you're just saying something about it, hence: la fiesta fue divertida. However if you were saying "the party was fun" as part of a narrative about something that happened at the party, and the fact of it being fun was part of the backdrop of your story, the imperfect would be correct: la fuesta era divertida, la gente bonita y alegre, y la música buena y a todo volumen. Fue entonces que .... (then something happened).

The main reason is that, when someone described a particular party (la fiesta), it was generally a one-time event in the past, and thus the past definite (pretérito, i.e., "fue") was used in the case. On the other hand, if one told a story on what was happening during the party, the imperfect (imperfecto, i.e., "era") would possibly be used, since it described the things happened within an indefinite past (during the party). === Deseo que tengas un buen día.／I wish you a good day.

the difference is time. Era is imperfecto and fue is Preterito. Preterito is an action in the past that was made. It started at some point in the past and it was complete. Imperfecto is an action in the past also, but we don't know when it started or it was finished. So in your question you are not talking about the description you are talking about something it was made. But if you are describing a story it could be the imperfect like "La fiesta fue divertida, hasta que se termino la comida" Here something happened that it wasn´t possible to continue the party Divertida is not a descripcion is a qualifying adjective, it is qualifying the noun "fiesta" and not the verb. Keep going!!

The two forms represent different past tenses, "era" for the imperfect and "fue" for the preterite. the imperfect tense generally refers to actions that occurred numerous times and/or didn't have a definite end, while the preterite typically refers to actions that took place or at least ended at a definite time. So in your sentance la fiesta fue divertida, it is used "fue" because the action ended at a definite time. If you were to use the impefect "ERA" in this phrase you have to add information to the sentance. For example: La fiesta era divertida, pero llego la policia y la arruino. (the party was fun, but the police came and ruined it) I hope that helps you! Saludos!