A fiesta is a celebration or a party. Your parents might plan a fiesta to celebrate your high school graduation.
You can use the word fiesta for any party, but it usually refers to a large feast, festival, or very extravagant party. In Spanish, fiesta means "feast," and in many Spanish-speaking places, a fiesta celebrates a saint's day or other religious occasion. The word comes from the Latin festus, "festive, joyful, or merry."