What does the 'D' in D-Day stand for?

The short answer: nothing.

In military terms, D-Day and H-Hour are sometimes used for the day and hour on which a combat attack or operation is to be initiated. They are used when the day and hour have not yet been determined, or when secrecy is important.

When used in combination with plus or minus signs, these terms indicate the length of time preceding or following a specific action. Thus, H-3 means 3 hours before H-Hour, and D+3 means 3 days after D-Day. H+75 minutes means H-Hour plus 1 hour and 15 minutes.

D-Day for the invasion of Normandy was set for June 6, 1944, and that date has been popularly referred to by the short title "D-Day."