The Collaborative International Dictionary of English v.0.48:

D-day \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]

WordNet (r) 3.0 (2006):

D-day
    n 1: date of the Allied landing in France, World War II [syn:
         D-day, 6 June 1944]