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.
This entry comes from Encyclopædia Britannica Concise.
For the full entry on D-Day, visit Britannica.com.
Seen & Heard
What made you look up D-Day? Please tell us what you were reading, watching or discussing that led you here.