Definition of D-Day noun from the Oxford Advanced Learner's Dictionary

 

D-Day

 noun
noun
BrE
 
; NAmE
 
[uncountable]
 
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a date on which something important is expected to happen From the name given to 6 June 1944, the day on which the British, US and other armies landed on the beaches of northern France in the Second World War. Culture The Allies were led by General Eisenhower, and defeated the German forces defending the beaches. The secret name of the landings was 'Overlord'. They are often also called the Normandy landings. The ‚D‘ is short for ‚day‘. In the First World War H-hour was used for the time of a planned attack. see also Mulberry harbours
See the Oxford Advanced American Dictionary entry: D-Day