What does "D" in D- Day stand for?

D = Day. The military uses the term "D-Day" as the first day of any major military operation. They use the term so as not have to changes to all there planning is certain conditions are not met. For example: D-Day on France in 1944 was originally set for June 5, 1944 but the weather was so bad that they to post pone the invasion by 24 hours. Note: The military uses H-Hour as the time the invasion is to start. For example H-Hour on Omaha Beach was 06:30.