What does the D stand for in D-Day ?

The D in D-Day is the same than the H in H-Hour.
in French, it give the Jour-J or the Heure-H.

They reused the first letter of the word in order to say : THE moment. Here, in the case of the landing on Normandy, it means THE moment of the attack. It's a code name and thanks to it we don't need to say the real date : we can say : D-Day + 5 for : the date of the attack + 5 days.