Q. What does the D in D-Day stand for?
A. "Day" - (in the same way as the H in H-Hour stands for Hour).
In military terms when an operation is planned the days leading up to the day of execution of a plan are labelled as, for example, D-10, D-9, D-8, D-7 and so on, meaning "Day Minus 10, Day Minus 9" etc. right up to D-Day itself. Similarly the days after an operation are labelled as D+1, D+2, D+3 etc. This system is used for two main reasons, one it helps to keep the actual planned date of an operation secret, and secondly that date can be changed without having to re-issue all the associated plans. In the same way in military planning, the hours leading up to and following an operation are labelled, H-4, H-3, H-2, H-1, H-Hour, H+1, H+2, and so on.
Thus the "D" in D-Day stands for "Day" despite it looking strange, and the H in H-Hour stands for "Hour". So you will see that there have actualy been many D-Days in military history - it's just because this one was so special it has retained its label.