From The Collaborative International Dictionary of English v.0.48 [gcide]:
Fiesta \Fies"ta\, n. [Sp. See Feast, n.]
1. Among Spanish, a religious festival; a saint's day or
holiday; also, a holiday or festivity.
Even . . . a bullfight is a fiesta. --Am. Dialect
Notes.
Some fiesta, when all the surrounding population
were expected to turn out in holiday dress for
merriment. --The Century.
[Webster 1913 Suppl.]
2. an organized series of acts and performances.
Syn: festival, feast, fete.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) [wn]:
fiesta
n 1: an elaborate party (often outdoors) [syn: fete, feast,
fiesta]