zero hour

[zero hour] {n.} 1. The exact time when an attack or other militaryaction is supposed to start.

Zero hour for the bombers to take offwas midnight.

2. The time when an important decision or change issupposed to come; the time for a dangerous action.

It was zero hourand the doctor began the operation on the man.

On the day of thechampionship game, as the zero hour came near, the players grewnervous.