zero hour

American  

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.

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zero hour British  

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

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Etymology

Origin of zero hour

First recorded in 1915–20

Example Sentences

The panel has called his bluff at the zero hour, playing the other side of the same game.

From Los Angeles Times • Oct. 13, 2022

And considering that journalists have long used the platform to document the country’s suppression of press freedom, press outlets view Twitter’s case as a sort of zero hour for Indian free expression rights.

From Slate • Jul. 21, 2022

This is zero hour, and those who don’t do the right thing now won’t get a second chance.

From Seattle Times • Sep. 7, 2021

On the tour, crowds listened raptly at each stop as MoveOn’s Washington director, Ben Wikler, warned that zero hour was coming on the bill and tried out chants.

From Washington Post • Jun. 26, 2017

It is at the zero hour of the big bang and the ground zero of the black hole that the mathematical equations that describe our world stop making sense.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife