Knowing when to Use Preterite or Imperfect?

Hola! For the most part I understand when to use the imperfect (an action repeated often in the past), and when to use the preterite (a quick one time kind of thing). I would like to understand why the RS flashcard sentence "La fiesta *estuvo* buenisima" uses the preterite instead of the imperfect "La fiesta *estaba* buenisima"? Thanks for any clarity you might be able to provide (I've noticed the same problem in French, and have never really been able to figure out why). Muchas gracias! Jim

I think it's because the party has been completed and is not still ongoing. Can anyone confirm?