D-Day 60 years

Okay, possible explanations for D:
1.
When someone wrote general Eisenhower in 1964 asking the explanation, his
executive assistant Brigadier General Robert Schultz answered: "General
Eisenhower asked me to respond to your letter. Be advised that any
amphibious operation has a "departed date"; therefore the shortened term
"D-day" is used"...

2.
All major offensive operations were planned to commence on "D-Day at
H-Hour at M-Minute."