The US Army says: "The terms D-day and H-hour are used for the day and hour on which a combat attack or operation is to be initiated. They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential. The letters are derived from the words for which they stand, 'D' for the day of the invasion and 'H' for the hour operations actually begin." <http://www.army.mil/cmh-pg/faq/ddaydef.htm>
While it's never safe to argue with the US Army, a dictionary I checked says D-Day means Deployment Day. This source offers no other definition for H-Hour, but (knowing the military mind as I do) my suggestion for an alternative definition would be Happy Hour ;-)
Carl
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