Why 6/11?
TL;DR: Convergence of Forces. Go Time.
6/9: Show up to Party
6/11: Operation Overlord - Party Time
On June 6, 1944, after a year of meticulous planning conducted in secrecy by a joint Anglo-American staff, the largest combined sea, air, and land military operation in history began on the French coast at Normandy.
On June 11, the five landing groups met up, and Operation Overlord–the code name for the Allied invasion of northwestern Europe–proceeded as planned.