History Stories


Why Is It Called D-Day?

The meaning behind the "D" in D-Day has to do with military terminology.


According to the U.S. military, “D-Day” was an Army designation used to indicate the start date for specific field operations. In this case, the “D” in D-Day doesn’t actually stand for anything—it’s merely an alliterative placeholder used to designate a particular day on the calendar. 

READ MORE: D-Day Facts About the Epic Invasion

The military also employed the term “H-Hour” to refer to the time on D-Day when the action would begin. This shorthand helped prevent actual mission dates from falling into enemy hands, but it also proved handy when the start date for an attack was still undecided. Military planners also used a system of pluses and minuses to designate any time or day occurring before or after D-Day or H-Hour. 

For example, D+2 meant two days after D-Day, while H-1 referred to one hour before H-Hour. These terms allowed units to effectively coordinate their operations ahead of time even when they didn’t know their actual start date, and they also provided flexibility in the event that the launch day shifted.

Use of these terms stretches back to World War I. One American field order from September 1918 noted, “The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient.” Other nations had their own shorthand. In World War I, the French used the code date “le Jour J,” while the British called their operation start days “Z-Day” and “Zero Hour.”
