When were D-Day and H-Hour first used to designate the starting time for a military operation?

answer

The St. Mihiel Offensive of the U.S. First Army, September 12, 1918

At an Allied conference on 24 July 1918, General Pershing chose the St. Mihiel sector for the first American offensive. The objective of the offensive was a salient projecting 16 miles into the Allied line. Roughly shaped like a triangle, the salient ran from Verdun on the north, south to St. Mihiel and then east to Pont-a-Mousson on the Moselle River. It was bordered by a line of hills known as the Heights of the Meuse and a succession of marshes and lakes situated across deep ravines and dense forests. In addition to its natural defensive advantages, the salient protected the stragegic rail center of Metz and the Briey iron basin so vital to the Germans as a source of raw material for munitions. Offensively, it interrupted French rail communications and constituted a constant threat against Verdun and Nancy. Reduction of the salient was imperative before any large Allied offensive could be launched against Briey and Metz or northward between the Meuse River and Argonne Forest. At the conference, General Pershing insisted that the attack be a United States Army operation with its own sector, under the separate and independent control of the American Commander- in-Chief.


Town of St. Mihiel on River Meuse
When the decision was made, there were over 1,200,000 American soldiers in U.S. troop units widely scattered throught France, either serving with French or British Armies or training in rear areas. In view of the splendid record that so many of the U.S. units had already achieved in combat, the Allies were forced to agree that a separate U.S. Army should be formed, although they requested that U.S. divisions continue to be permitted to fight with their armies.

The order creating the United States First Army became effective on 10 August 1918. On 30 August, the U.S. First Army took over the St. Mihiel sector. After a series of conferences, the Allies agreed that the St. Mihiel attack should be limited to a reduction of the salient, following which the U.S. First Army would undertake a larger scale offensive on the front between the Meuse River and the Argonne Forest. With the attack at St. Mihiel scheduled for 12 September, this would require winning an extraordinarily swift victory there, then concentrating an enormous force to launch a still greater operation 40 miles away, within just two weeks. Never before on the western front had a single army attempted such a colossal task.

At 0500 hours (H-Hour), 12 September 1918 (D-Day), following a four-hour bombardment by heavy artillery, the U.S. I and IV Corps composed of nine U.S. divisions, began the main assault against the southern face of the salient, while the French II Colonial Corps made a holding attack to the south and around the tip of the salient. A secondary assault by the U.S. V Corps was made three hours later against the western face of the salient. Reports were soon received that the enemy was retreating. That evening, the order was issued for U.S. troops to press forward with all possible speed. By the dawn of 13 September, units of the U.S. IV and V Corps met in the center of the salient, cutting off the retreating enemy. By 16 September, the entire salient had been eliminated. Throughout these operations, the attacking forces were supported by the largest concentration of Allied aircraft ever assembled. The entire reduction of the salient was completed in just four days by which time some of the divisions involved had already been withdrawn to prepare for the Meuse-Argonne battle.

From:  The Website of the American Battle Monuments Commission