H-hour is a term which indicates the moment a (militairy-) operation will take place. also called D-day, although the term D-day mainly refers to the landing of allied forces in Normandie France, second world war.

hope to 've been of help.

p.s.
although mainly used in the army it is also possible to use it for other planned operations or actions of importancy. In dutch its possible to use " D-day " instead of H-hour.

f.i.: its even possible to say something like: " Tomorrow at three its H-hour for me at the dentist." If this event has a great impact on the person. The term is than used to stress the importancy of the event.