zerohour
nounUK /ˈzɪərəʊ aʊə/
US /ˈzɪroʊ aʊr/
The exact time when something important is planned to start.
The rocket launch is set for zero hour at dawn.
OTHER MEANINGS
2. The time when a military attack or operation is set to begin.
The soldiers waited quietly for zero hour.
3. A critical or decisive moment when action must be taken.
For the team, zero hour came when the clock hit five minutes left.
4. The scheduled time for a planned event or activity to start.
Zero hour for the school play is seven o'clock tonight.