The D in D-Day is the same than the H in H-Hour. It refers to a military vocabulary. .It is also possible to say : Y-Year.

In French, the army uses the same system and it gives: the Jour-J, the Heure-H or the Année-A.

Soldiers use the first letter of the word in order to say : THE moment. Here, in the case of the landing of Normandy, it means THE moment of the attack. It is a code name and thanks to it is no need to say the exact date. Thus it is possible to say “D-Day + 5” for : the date of the attack + 5 days.

This code avoids the ennemy intelligence to find too easily the right date or the right hour of the offensive.