"D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar terms. The initial D in D-Day has had various meanings in the past, while more recently it has obtained the connotation of "Day" itself, thereby creating the phrase "Day-Day", or "Day of Days". On the same principle, the equivalent terms in French, Basque and Romanian are Jour J, E eguna and Ziua-Z.
The best known D-Day is June 6, 1944 — the day of the Normandy Landings— initiating the Western Allied effort to liberate mainland Europe from Nazi occupation during World War II.
However, many other invasions and operations had a designated D-Day, both before and after that operation.
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.
D-Day 6/6/44
FDR D-Day Speech June 6, 1944
Normandy Speech: Ceremony Commemorating The 40th Anniversary Of The Normandy Invasion, D-Day 6/6/84
Saving Private Ryan D-day Scene (3/4)
Twelfth Army situation map for 2400 hours, 6 June 1944.
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.
Dwight D Eisenhower to delay until June 6 and that date has been popularly referred to ever since by the short title "D-Day". For example, Douglas MacArthur's invasion of Leyte began on "A-Day", and the invasion of Okinawa began on "L-Day".