D for Day, H for Hour are abbreviations for the undetermined (or secret) day and hour that the Normandy Invasion would begin. It’s U.S. military terminology allowing for precise planning without giving away the date and time. The French used the code date “le jour j” and the British called it “Z-Day” and “Zero-Hour.”There were actually two D-Days. The first took place Sept. 7, 1918, when the US First Army led by General Pershing was to attack at H hour on D day with the object of forcing the evacuation of St. Mihiel salient. The salient was a ridge near ...