What does the D in D-Day stand for?
Not a blooming thing. Every major operation had a D-day and an H-hour that marked when it was scheduled to begin. Since these things were planned way in advance, D-day and H-hour were entered into communications as substitutes for the date and time of the operation.The proceedings of operations were planned starting from D-day H-hour or 0-hour and continuing from there with D+1 day, D+2 day, etc. Since many operations were often dependant on favorable weather (among other things), the date of a maneuver was sometimes not known until the very last minute, thus the usefulness of the term D-Day.
The expressions D-day and H-hour were first used during the first world war on September 20, 1918, when the US First Army issued Field Order No 8 which read, "The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel Salient."
Because Operation Overlord was so significant, the term D-Day has become synonymous with the Allied landings in Normandy of June 6, 1944.