Memorial Day is a federal holiday in the United States for mourning the U.S. military personnel who died while serving in the United States Armed Forces. It is observed on the last Monday of May. It is the unofficial beginning of summer in the United States. Memorial Day is a time for visiting cemeteries and memorials to mourn the military personnel who died in the line of duty. Volunteers will place American flags on the graves of those military personnel in national cemeteries.