Some would say the "D" of D-day stands for "designated". I've also heard the theory that it stands for the first letter of "_der_ Tag" i.e. _the_ day (in German). However, the official interpretation is that the "D" simply stands for day (i.e that day of the invasion) in the same way that H-hour stands for the _hour_ of invasion, and M-minute even the _minute_ of invasion [D-day was a very time critical operation]