How was D-Day planned and executed?

The answer

Operation Overlord on June 6, 1944, involved 156,000 Allied troops, 5,000 ships, and 11,000 aircraft - the largest amphibious invasion in history. It was postponed by one day due to storms, and Eisenhower even drafted a message accepting blame in case of failure. Crucially, Operation Fortitude - an elaborate deception using inflatable tanks, fake radio traffic, and double agents - convinced Hitler that the real invasion would come at Calais, not Normandy, causing him to withhold reinforcements during the critical first hours.