The Collaborative International Dictionary of English v.0.48:

Gala \Ga"la\, n. [F. gala show, pomp, fr. It. gala finery, gala;
   of German origin. See Gallant.]
   Pomp, show, or festivity. --Macaulay.
   [1913 Webster]

   Gala day, a day of mirth and festivity; a holiday.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gala
    n 1: a gay festivity [syn: gala, gala affair, jamboree,
         blowout]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "gala":
   Mardi Gras, Saturnalia, ball, banquet, blowout, carnival,
   celebration, celebratory, cheerful, cheery, convivial, do, event,
   fair, feast, festal, festival, festive, festive occasion,
   festivity, fete, field day, fiesta, gala affair, gala day, gay,
   gladsome, gleeful, great doings, happening, happy, high jinks,
   hilarious, holiday, holy day, jamboree, jolly, jovial, joyful,
   joyous, kermis, merry, merrymaking, occasion, on the loose,
   pageant, party, picnic, red-letter day, waygoose, wayzgoose