D-day

n. 1. (Mil.) 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. [ PJC ]

2. 6 June 1944, the day of the landing in France, by troops allied against the Germans in World War II.
Syn. -- 6 June 1944. [ WordNet 1.5 ]

3. the day for commencement or execution of any elaborate planned activity; -- often used jocosely. [ informal ] [ PJC ]