GNU Collaborative International Dictionary of English

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Found one definition

1.       
   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]