What is the origin of the idiom "zero hour" and when to use it?

The origin of the idiom "zero hour" can be traced back to military terminology. It was initially used in the context of military operations and combat planning. The term refers to the specific time when an operation or attack is scheduled to commence. The concept of "zero hour" emerged during World War I and was commonly used during subsequent conflicts. It is used to describe critical moments or decisive actions in various contexts, including military operations, project management, emergencies, and personal choices, signifying the pivotal point when important events or decisions take place.