ZERO HOUR

{n.} 1. The exact time when an attack or other military action is supposed to start. * /Zero hour for the bombers to take off was midnight./ 2. The time when an important decision or change is supposed to come; the time for a dangerous action. * /It was zero hour and the doctor began the operation on the man./ * /On the day of the championship game, as the zero hour came near, the players grew nervous./