D-Day was the invasion of the Normandy, France shores on June 6, 1944. It is generally thought of the beginning of the end for the Nazi regime during World War I. As far as what it actually means, the military code for the first day of an invasion is usually "D" but it has also been suggested to stand for “disembarkation day" or “decision day”. That is still up in the air.
D-Day has come to mean the one invasion by the Allies into Axis occupied Europe on June 4,1944. However, the term, which originated from military planners during World War I, denotes the commencement of any military operation. Additionally, there are M-day, which is the day where mobilization occurs, C-day, when deployment occurs, and other "lettered" days that define a military action. Se more at the link:
It is also called Operation Overload. just fyi.
The D day is the day in which the Antifascist Coalition landed in Normandy and it was the beginning of Europe's freedom from Hitler's domination.