The Normandy invasion was a very large scale and well planned invasion during WWII. There were three key periods of the invasion: D-day, fighting inland, and the breakout. The Normandy invasion was a major turning point in WWII, and probably the biggest as well.
D-day was definitely the most famed and important part of the invasion. It was important in the sense that it got the troops into Normandy, but it really didn't mark the turning point yet. It was the "kick-off- of the invasion. Without the success of D-day, the Allied troops might not have pushed the Axis forces out of France.
There were many factors leading to the success of D-day. For one, the weather was horrible, and although this made for bad beach landings, the Axis had a hard time finding targets in the storm (Battle of Normandy 410). The Axis did know that the Allies were to invade at some point, but they were caught off guard when the invasion took place during a storm, and because of this there were less Axis troops at the beaches of Normandy that day (Battle of Normandy 410). German High command's leader was home on leave during the invasion, making troops less organized (Battle of Normandy 410). Probably the most helpful was the fact that there were 12,000 allies bombers/fighters circling the skies during the D-day, letting lieutenants call in air strikes at certain strategic positions (Battle of Normandy 410). Once the invasion was over, the Allies used the newly captured Axis fortress as a drop-off for supplies, weapons, and new soldiers and armoured divisions.
D-day angered Hitler, so he launched quick, poorly planned assaults soon after the initial invasion to push the Allies out. He failed all attempts. The Allied forces pushed inland and a stalemate in the invasion had begun. Neither side was getting enough reinforcements or supplies to push the other back, so the fighting stayed confined inside areas of Normandy.