On This 6th Day of June, 1944

Allies faked a invasion of Calais coast German fell for it.
Hitler also refused a number of suggestion to defend Normandy from his top generals most notably Erwin Rommel wanting to move more Tank division into range of the coast Hitler refused and kept them in Paris.

Allies played Hitler incompetence / ego quite well.

America took it the hardest on Normandy, Canada got very lucky just a few miles down the beach.