D-Day refers to June 6, 1944, the day on which the Allies invaded Nazi occupied France during World War II. The landings took place along a 50-mile stretch of the Normandy coast divided into five sectors: Utah, Omaha, Gold, Juno and Sword. In more general military parlance, D-Day and H-Hour are used for the undetermined day and hour on which a combat attack or operation is to be initiated, usually where secrecy is essential.