The 'D' in D-Day simply stands for 'Day'.
Why it is true
It is a military term used to denote the day on which a combat attack or operation is set to begin. The 'H' in H-Hour stands for 'Hour' for the same reason.
Source: Military History
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.