Why is D-Day Jour de J in french?

The terms "H-Hour" and "D-Day" comes from the military's term for the hour and day when an attack will occur, and as such, it is a variable. (It is used instead of an exact date when either the date is unknown, or secrecy is required.) Therefore, I assume that the French call the D-Day invasion of Normandy "Le Jour J" for the same reason; the "J" is the variable for the "jour" (day) of the attack.