Stalin wanted an invasion in western Europe so the germans could not concentrate all their forces against Russia. The invasion of Italy didn't satisfy him, there were to few forces apposing the allies, and Italy was to narrow to make an effective battleground. Besides the Brittish and the Americans wanted to liberate France, and they also wanted to go against Germany. The solution to this problem was the invasion of Normandy.
The brittish and the americans used a lot of time to ready themselves for the invasion, and in all this time Hitler had to send more and more soldiers away from the russian front to counter the ever growing number of troops preparing for the invasion. This was perhaps more important than the invasion itself, as these german troops could have changed the outcome of the battle of Stalingrad or Kursk, while at the time where the invasion took place, the germans were already retreating at the russian front. This is just my personal oppinion.
Commander of the german forces was Erwin Rommel, Rommel meant that the invasions outcome would be decided on the beach, the first day of the invasion. So he filled the beaches with mines, tank-barriers, and barriers that could rip open the bottom of the boat.
German intelligence and Hitler thought that the invasion would take place at Calais, and they kept that oppinion even after the invasion had taken place. That is why Rommel was denied reenforcements, and that is probably why the invasion succeded.
The correct invasion came at Normandy, covered by heavy airpower and the navys battleships, american, brittish and canadian forces stormed the beaches and siezed a narrow stretch of land. Rommel had said that the first 24 hours would decide the fate of Germany and the invasion, however during the first day of the invasion Rommel was away visiting his family. However it took considerable time to expand that strip of land.