the day on which a military attack or other important event is to take place; specif., June 6, 1944, the day of the invasion of W Europe by Allied forces in WWII
See D-day in American Heritage Dictionary 4
(dēˈdāˌ)
noun
The unnamed day on which an operation or offensive is to be launched.
The day on which the Allied forces invaded France during World War II (June 6, 1944).