Memorial Day (originally known as Decoration Day) is one of the federal holidays in the United States for honoring and 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. Memorial Day is also considered the unofficial beginning of summer in the United States. It is a day 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.
https://en.wikipedia.org/wiki/Memorial_Day