Memorial Day in the U.S. commemorates those who have died while serving in the military. Observed on the last Monday of May, it originated after the Civil War as "Decoration Day," when citizens honored fallen soldiers by placing flowers on their graves. Over time, Memorial Day evolved to honor all Americans who died in military service, especially during World War I, World War II, the Vietnam War, and more recent conflicts. It serves as a day of remembrance, reflection, and recognition of the sacrifices made by service members, often marked by ceremonies, parades, and the decoration of graves with flags and flowers.