d day

time of military action, date and time set for a military operation; 6th of June 1944, date on which Allied troops invaded the beaches of Normandy France and fought in heavy battles against Nazi troops

n. time of military action, date and time set for a military operation; June 6th 1944, date on which Allied troops invaded the beaches of Normandy in France and fought in heavy battles against Nazi troops

D-Day may also refer to Decimal Day in the United Kingdom. D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day".
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Noun
1. day of the allied landing in France, World War II
(synonym) 6 June 1944
(hypernym) date, day of the month

D-day, time of military action, date and time set for a military operation

n. D-day