D-Day commonly refers to the Normandy landings, which took place on June 6, 1944. The landings by Allied forces on the beaches of Normandy in France were amongst the largest seaborne invasions in history and formed a key component of the liberation of northwestern Europe from Nazi control.
D-Day is a military term used to indicate the day on which a particular operation or military attack is to begin. The "D" is a placeholder that indicates a given date on the calendar. The use of D-Day as a military term provides a way of avoiding the details of an operation from falling into enemy hands and is also convenient for discussing operations that have no designated start date. H-Hour is used in the same way to indicate the time at which an operation begins.