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]