D DAY. The term "D Day" indicates the beginning of an attack or other military operation when the specific date has yet to be selected or secrecy is required. "H Hour" is similarly used to designate the time of the attack.
The "D" and "H" are derived from the first letters of "day" and "hour." There is one D Day and H Hour for all units participating in an operation. Plus and minus signs are used to indicate the number of days or hours that precede or follow the specific operation. Thus, 舑5 means five days before D Day and H + 2 means two hours after H Hour.
Planning for operations can begin months before the anticipated time of the operation. The use of D Day minus "X number of days" signifies the date by which certain actions, such as planning or the training of units, must be complete. At the appropriate time an order is published giving a specific date for D Day.
The U.S. Army first used the term on 7 September 1918, when it issued First Army Field Order Number 9: "The First Army will attack at H Hour on D Day with the object of forcing the evacuation of the St. Mihiel Salient." The term is most commonly associated with the invasion of Normandy on 6 June 1944.
See alsoNormandy Invasion .