In U.S. military history, any designated day for the commencement of a major operation.
The designation apparently originated in World War I; its meaning is uncertain, though it is probable that the "D" stands for nothing more than "Day." (The designated time for commencement of action on any D-Day was referred to as H-Hour.) The most celebrated D-Day occurred on June 6, 1944, the first day of the Anglo-American invasion of Europe in World War II. See Normandy Campaign .