|1.||the day, June 6, 1944, on which the Allied invasion of Europe began|
|2.||the day on which any large-scale operation is planned to start|
|[C20: from D(ay)-day; compare |
"The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient." [Field Order No. 8, First Army, A.E.F.]"They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential" [U.S. Army Center of Military History Web site]. Now almost exclusively of June 6, 1944.
The code name for the first day of a military attack, especially the American and British invasion of German-occupied France during World War II on June 6, 1944 (see invasion of Normandy). This marked the beginning of the victory of the Allies in Europe. Germany surrendered less than a year later.