zero hour
1．the exact time when an attack or other military action is supposed to start 发动攻击的时间；采取军事行动的时间
The company was told that their zero hour was 3 a．m．连队奉命于凌晨3点开始行动。
2．the time when an important decision or change is supposed to come；the time for a dangerous action 做出重要决定或重要变动的时间；关键时刻；危急关头
It was zero hour and the doctor began the operation on the patient．那是个关键的时刻，医生开始为那个病人动手术了。