zero hour

noun

1. the time set for the beginning of a military attack or operation.

2. the time set for the beginning of any event or action.

3. a decisive or critical time.

zero hour

noun

1. military the time set for the start of an attack or the initial stage of an operation

2. informal,  a critical time, esp at the commencement of an action

Word History and Origins

Origin of zero hour¹

First recorded in 1915–20