WordNet (r) 3.0 (2006):

happy hour
    n 1: the time of day when a bar sells alcoholic drinks at a
         reduced price

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "happy hour":
   break, breath, breather, breathing place, breathing space,
   breathing spell, breathing time, cigarette break, cocktail hour,
   coffee break, downtime, enforced respite, halt, interlude,
   intermission, letup, lull, pause, recess, respite, rest, spell,
   stay, surcease, suspension, tea break, time out