Definitions for d-dayˈdiˌdeɪ
Random House Webster's College Dictionary
a day set for beginning something.
June 6, 1944, the day of the invasion of W Europe by Allied forces in World War II.
Origin of D-day:
1915–20; D (for day) +day
D-day, 6 June 1944(noun)
date of the Allied landing in France, World War II
June 6, 1944, the date during World War II when the Allies invaded western Europe.
The date of any major event planned for the future.
Dictionary of Military and Associated Terms