The greatest amphibious assult in war history happened on June 6, 1944.
In military parlance, D-Day is a term often used to denote the day on which a combat attack or operation is to be initiated. By far the most well-known D-Day is June 6, 1944—the day on which the Battle of Normandy began—commencing the British, American, and Canadian liberation efforts of mainland Europe from Nazi occupation during World War II. This article discusses the general use of the term D-Day. Refer to the Battle of Normandy article for a description of the events of June 1944.
The terms D-day and H-hour are used for the day and hour on which a combat attack or operation is to be initiated. They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential. There is but one D-day and one H-hour for all units participating in a given operation.
When used in combination with figures, and plus or minus signs, these terms indicate the point of time preceding or following a specific action. Thus, H-3 means 3 hours before H-hour, and D+3 means 3 days after D-day. H+75 minutes means H-hour plus 1 hour and 15 minutes. Planning papers for large-scale operations are made up in detail long before specific dates are set. Thus, orders are issued for the various steps to be carried out on the D-day or H-hour minus or plus a certain number of days, hours, or minutes. At the appropriate time, a subsequent order is issued that states the actual day and times.
The earliest use of these terms by the U.S. Army that the Center of Military History has been able to find was during World War I. In Field Order Number 9, First Army, American Expeditionary Forces, dated 7 September 1918: "The First Army will attack at H hour on D day with the object of forcing the evacuation of the St. Mihiel Salient."
D-day for the invasion of Normandy was originally set for June 5, 1944, but bad weather caused Gen. Eisenhower to delay until June 6, and that date has been popularly referred to ever since by the short title "D-day". (In French, it is called jour-J.) Because of this, planners of later military operations sometimes avoided the term. For example, MacArthur's invasion of Leyte began on "A-day", and the invasion of Okinawa began on "L Day". November 1, 1945, the proposed date for an invasion of the Japanese home islands, was to be "X-Day". A second wave of landings near Tokyo were set down for "Y-Day", March 1, 1946.