zero-hour

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.

Origin:
1915–20

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
zero hour
 
n
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

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