The 'D' in D-Day does not stand for anything, it's a just a name the military use when planning an event.
The military started planning the details of the invasion before they knew the date when it would take place. In order to organise things like when the troop ships should leave England they referred to it as D-Day.
Here's how it worked
In May 1944 they couldn't say:
"Captain Smith will set sail for France on 5 June" because they didn't yet know that the invasion was on 6 June.
So they said:
'Captain Smith will set sail at D-Day minus one.' Then all Captain Smith had to do was wait until someone told him the date of D-Day. He just deducted one day to work out the date he should sail for France.
The name D-Day has been used for many military operations, but it is now firmly associated with the Allied invasion of Normandy.