The Answer:

Nothing. Or maybe "Day."

It is widely believed that the "D" in D-Day stands for "designated," or "decision," or something else along those lines. These beliefs, while creative, are unfounded. According to the Department of Defense Dictionary of Military and Associated Terms, D-Day is simply "The unnamed day on which a particular operation commences or is to commence." Similarly, H-Hour is "The specific hour on D-day at which a particular operation commences."

See the full entry on "times" for several other, more recent, codes.

D-Day is commonly used to describe June 6, 1944, the day Allied forces executed a plan called Operation Overlord to invade France during World War II.

You can find more information at the National D-Day Memorial Foundation site.

—The Editors