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>as, the day before <ex>D-day</ex> the troops will move into position</as>.
• ⇰ 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.