It is actually and abbreviation of the phrase Day of Days, first used in World War I by the American Expeditionary Force in 1918, and this works with all languages that use this code, Jour J in French, E eguna in Basque, Ziua-Z in Romanian, and Dan D in Slovenian.
D-Day stands for DEBARKATION DAY...this is from a World War II veteran the letter D. D-Day was just a name given to the day on which a particular operation commences or is to commence. For an hour they would call it H-Hour, for a day D-Day. There is actually an entry in the military times with all of these classifications. C-day, M-day, F-hour, H-hour, all of these names are there. All just classifications for an unnamed day in which action was to take place.
Even better, important assaults are called D-Day because the D at the beginning means Day, so the word means Day-Day, or Day of Days and the same for H-Hour
Note: There are comments associated with this question. See the http://wiki.answers.com/Q/Discuss:What_does_the_D_in_D-Day_stand_for to add to the conversation.