The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

fiesta
    n 1: an elaborate party (often outdoors) [syn: fete, feast,
         fiesta]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "fiesta":
   Mardi Gras, Saturnalia, banquet, blowout, carnival, do, fair,
   feast, festival, festive occasion, festivity, fete, field day,
   gala, gala affair, gala day, great doings, high jinks, jamboree,
   kermis, party, picnic, waygoose, wayzgoose