Definitions for "D-day"
the day designated for the beginning of a planned attack by a military force; in communications the day is often otherwise unspecified for security reasons; as, the day before D-day the troops will move into position.
6 June 1944, the day of the landing in France, by troops allied against the Germans in World War II.
the day for commencement or execution of any elaborate planned activity; -- often used jocosely.