The Allies tried to conceal the true location for the D-Day landings by doing the following:
1) set up a huge dummy army with its own headquarters and equipments.
2) created fake documents where their intentions were documented and made sure that the Germans intercepted these documents.
3) passed on "secret" information to known German agents
4) the dummy army showed that they intended to attack the French seaport of Calais
These maneuvers were so successful that the Germans did not respond to the Allies landing on Normandy, thinking that these landings were intended to distract them.
