Memorial Day (originally known as Decoration Day) is a federal holiday in the United States for mourning the U.S. military personnel who have died while serving in the United States armed forces. It is observed on the last Monday of May. From 1868 to 1970, it was observed on May 30.

Many people visit cemeteries and memorials on Memorial Day to honor and mourn those who died while serving in the U.S. military. Many volunteers place American flags on the graves of military personnel in national cemeteries. Memorial Day is also considered the unofficial beginning of summer in the United States.

The first national observance of Memorial Day occurred on May 30, 1868. Then known as Decoration Day, the holiday was proclaimed by Commander in Chief John A. Logan of the Grand Army of the Republic to honor the Union soldiers who had died in the Civil War. This national observance was preceded by many local ones between the end of the Civil War and Logan's declaration. Many cities and people have claimed to be the first to observe it. However, in 2022, the National Cemetery Administration, a division of the Department of Veterans Affairs, credited Mary Ann Williams with originating the "idea of strewing the graves of Civil War soldiers - Union and Confederate" with flowers.

Official recognition as a holiday spread among the states, beginning with New York in 1873. By 1890, every Union state had adopted it. The World Wars turned it into a day of remembrance for all members of the U.S. military who fought and died in service. In 1971, Congress standardized the holiday as "Memorial Day" and changed its observance to the last Monday in May.

Two other days celebrate those who have served or are serving in the U.S. military: Armed Forces Day (which is earlier in May), an unofficial U.S. holiday for honoring those currently serving in the armed forces, and Veterans Day (on November 11), which honors all those who have served in the United States Armed Forces.