The Normandy landings were the beginning of Operation Overlord - or the invasion of German-occupied Europe.
Originally planned to take place on 1 May 1944, the operation was postponed a month to allow time to gather more troops and equipment. The timing was important to allow for the right weather, a full moon, and tidal conditions.
To keep the destination of the landings secret, a deception plan Operation Fortitude was mounted which led the Germans to believe the main target was the Pas de Calais, much farther east.
When the landings finally began there were only 14 of the 58 German divisions in France facing the Allies. While there was stiff resistance at other beaches, Omaha was the only one where the success of the Allied mission was in serious doubt.
The invasion of Normandy was the largest amphibious assault ever launched. It involved five army divisions in the initial assault and over 7,000 ships. In addition there were 11,000 aircraft.
In total 75,215 British and Canadian troops and 57,500 US troops were landed by sea on D-Day. Another 23,400 were landed by air....(BBC here)
There is much more (and GREAT videos) over at Assoluta Tranquillita here.