WordNet (r) 3.0 (2006):

zero hour
    n 1: the time set for the start of an action or operation

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "zero hour":
   A-day, D day, H hour, charged moment, contingency, crisis,
   critical moment, crossroads, crucial moment, deadline,
   decisive moment, eleventh hour, emergency, exigency, kairos,
   kairotic moment, moment of truth, nick of time, pass, pinch,
   pregnant moment, psychological moment, strait, target date,
   target day, turning point