Why did D-Day take place? Simple. The Allies needed to both: A. Take back France, and B. Find a route of invasion into Nazi-occupied Europe. The northern coast of France, named Normandy, was the perfect spot. Thus, the D-Day invasion will take place here.
D-day was the code name chosen for the invasion of France by the Allies during WWII. It meant the day that the invasion operation was to take place. The following days would be D-day plus one, D-day plus two, and so on.