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

See the Department of Defense Dictionary of Military Terms (linked below)

The DOD uses letters of the alphabet to specify different hours or days. Here are some examples:

C-day is the day on which a military deployment is scheduled to begin. That is, C-day is when troops currently stationed in the US are to be deployed for service abroad (or vice versa). 

D-day specifies the day on which a military operation begins. Although when we hear D-day we usually think of WWII, every battle has its D-day. For instance, D-day for Operation Desert Storm was Dec. 2, 1990. 

M-day is the specific day on which a mobilization is set to begin. Mobilization is the act of assembling and getting troops and supplies ready for war. Mobilization begins before deployment.

There are also N-days, R-days, S-days, T-days, and W-days. Visit the link below for information about them.