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.
I have searched high and low for this; however, the explanation seems to be a simple one. Each website I consulted referred to the "D" in D-Day as simply standing for "day," which makes NO sense, I know. These critics simply say that D-Day simply referred to the day of the invasion and nothing more.
...the most ordinary and likely of explanations is the one offered by the U.S. Army in their published manuals. The Army began using the codes "H-hour" and "D-day" during World War I to indicate the time or date of an operation's start. Military planners would write of events planned to occur on "H-hour" or "D-day" -- long before the actual dates and times of the operations would be known, or in order to keep plans secret. And so the "D" may simply refer to the "day" of invasion. ("American Experience: D-Day")
My grandfather grew up in a neighborhood in which all of the families had fathers who had served in the military during World War II, who had gone to school on the GI Bill, and who had purchased homes with a GI loan.
According to their collective opinion, D-Day was Deliverance Day, and H-Hour is Hit Hour.
As far as I know, none of the men are still alive to double check this; and after reading the postings on this thread, I'm guessing that their collective opinion is not the official definition, but this would not be the first time that the dictionary definition is different than the common use of a word.
D-Day is a millitary term which means a day where a millitary operation or an attack would be initiated. The "D" may just refer to the day of invasion. The most notable D-Day operation was june 6, 1944, the day where the Normady Landings took place, starting the Allied Powers effort to liberate mainland Europe from Nazi's Hilter control during World War II. H-hour is also used to describe the hour where the operation is going to start.
For example, a general would say, "I would attack Berlin at H-hour on D-Day with the objective of hitting Germany at its weakest"