The most widely acknowledged explanation for why that event is remembered as “D-Day” is a straightforward one. The “D” stands for “day.” “It simply signifies the day that the invasion will launch and puts all the timetables into play,” says Keith Huxen, Senior Director of Research and History at the National WWII Museum.

As early as World War I, the U.S. military used the term D-Day to designate the launch date of a mission. One reason was to keep the actual date out of the hands of spies; another was to serve as a placeholder until an actual date was chosen. They also used H-Hour for the specific time of the launch.