Wiktionary
n. (context figuratively English) The hour at which any major event planned for the future is set to begin, as used in United States military.
Wikipedia
H-hour was the name given to the airborne assault during the Normandy landings of World War II. The units involved included the U.S. 101st Airborne Division and U.S. 82nd Airborne Division, along with the British 6th Airborne Division. This took place about three hours before the main beach landings on the Normandy. The airborne invasion consisted of over 20,000 men and around 1,200 planes and gliders. The combined assault of the three Allied airborne divisions would surprise the German defenders and cause enough havoc behind the German lines, enabling the beach landings to go more smoothly. .
Usage examples of "h-hour".
The assumption was that there were six partially fortified 155-mm guns emplaced there, which probably would not have been destroyed in the pre-D-day bombing and in the heavy naval fire directed at them just before H-hour, The 2nd and 5th Ranger Battalions, attached to the 116th Infantry, had the special H-hour mission of taking out these guns.
Why not pre-enact evacuation, time it, then start the evacuation at minus H-hours plus thirty minutes?
As H-hour approached, Admiral Magruder could only watch the ponderous uncoilings of the many-headed beast he'd released, and pray that each head, each movement followed the plan worked out by Colonel Caruso, Admiral Simpson, and himself.