The historical event commemorated by Memorial Day in the United States is the sacrifice made by military personnel who have died in the line of duty while serving in the armed forces. Originally known as Decoration Day, it began in the aftermath of the Civil War to honor Union and Confederate soldiers. Observed on the last Monday of May, Memorial Day is marked by various traditions, including visiting cemeteries and memorials, holding parades, and flying flags at half-mast. Over the years, it has evolved to honor all U.S. military members who have died in wars, fostering a sense of national remembrance and gratitude for their service.