1. 6 June 1944, the day on which the Allies landed in Normandy, northern France, during World War II . They 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 .
2. a date on which something important is planned to happen:
As D-Day approached we still weren’t ready to move house.