D-Day

UK: /ˈdiːdeɪ/ US: /ˈdiˌdeɪ/ ,(dē′dā′)

WordReference Random House Learner's Dictionary of American English © 2026

D-day or D-Day /ˈdiˌdeɪ/   n. [uncountable]

1. Military a day set for beginning something, esp. June 6, 1944, the day of the invasion of W Europe by Allied forces in World War II.

WordReference Random House Unabridged Dictionary of American English © 2026

D-day  (dē′dā′), n. 

1. Military the day, usually unspecified, set for the beginning of a planned attack.
2. Government June 6, 1944, the day of the invasion of western Europe by Allied forces in World War II.
3. [Informal.] any day of special significance, as one marking an important event or goal.

Also, D-Day. 

• the same pattern as H-hour Dutch (for day) + day

Collins Concise English Dictionary © HarperCollins Publishers::

D-day noun

1. the day, June 6, 1944, in World War II on which the Allies invaded France in a series of beach landings in Normandy

Etymology: 20th Century: from D(ay)-day; compare H-hour