What does the “D” in D-Day stand for?
The “D” does not stand for "Deliverance", "Doom", "Debarkation" or similar words. In fact, it does not stand for anything. The “D” is derived from the word "Day". “D-Day” means the day on which a military operation begins. The term "D-Day" has been used for many different operations, but it is now generally only used to refer to the Allied landings in Normandy on 6 June 1944.
Why was the expression "D-Day" used?
When a military operation is being planned, its actual date and time is not always known exactly. The term "D-Day" was therefore used to mean the date on which operations would begin, whenever that was to be. The day before D-Day was known as "D-1", while the day after D-Day was "D+1", and so on. This meant that if the projected date of an operation changed, all the dates in the plan did not also need to be changed. This actually happened in the case of the Normandy Landings. D-Day in Normandy was originally intended to be on 5 June 1944, but at the last minute bad weather delayed it until the following day. The armed forces also used the expression "H-Hour" for the time during the day at which operations were to begin.
http://www.ddaymuseum.co.uk/