In fact, D-Day is a military term, which is used to describe a day of the beginning of some operation. Globally, the phrase is used to refer to one particular event, which is the the Allied landings in Normandy on the 6th of June, 1944.

And what does "D" actually stand for? Nothing, as it was just derived from the word "day". Some people also believe that the letter used to stand for the word "date". Anyway, all the possible meanings of the letter are connected with some particular or special day of the beginning of the military operation.

More Info: