The United States military uses the term D-day as a start date for field operations. There are many D-day dates in history but the most famous is the one that occurred on June 6, 1944.
Another code word for the beginning of an operation by the armed forces is H-hour. These two terms began to be in popular use during World War I.
The most famous D-day occurred in 1944. Operation Overload is another name for the event. This D-day began what was to result in the Allied liberation of Western Europe from Nazi control. The Battle of Normandy is also a name for the event.