"D-Day" is a term used to describe the day on which a military operation begins. It was first used in the context of the invasion of Normandy by Allied forces during World War II.
The specific origins of the term "D-Day" are not entirely clear, but it was commonly used by military planners and commanders to refer to the day on which a major military operation was scheduled to begin. The "D" in "D-Day" is simply an abbreviation that stands for "day," just as the "H" in "H-Hour" (the specific time at which an operation begins) stands for "hour."
The use of the term "D-Day" in the context of the Normandy invasion on June 6, 1944, became widely known and has since become synonymous with that particular event. The term was also used in other military operations during and after World War II.
Today, the term "D-Day" has taken on a broader meaning and is often used to refer to any significant event or turning point in history.