D-Day first means 6 June 1944. On that day in World War II, Allied soldiers — from the United States, Britain, Canada and others — crossed the sea and landed in Normandy, in northern France, to begin pushing into Europe against Nazi Germany. General Eisenhower was in charge. People still use D-Day in a looser way: the day a big, planned action finally starts — a launch, a move, an operation. The D does not stand for a secret word. It stands for day, the same way military people say H-hour for the exact hour a plan begins: you name the day before you pin down the calendar date. In history class, though, D-Day almost always means that June morning in 1944.