I'm just going to quote da wiki here regarding Memorial Day
Memorial Day (originally known as Decoration Day[1]) 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.[2] It is observed on the last Monday of May. It was formerly observed on May 30 from 1868 to 1970.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 an American flag on graves of military personnel in national cemeteries. Memorial Day is also considered the unofficial beginning of summer in the United States.
Many cities and people have claimed to have first celebrated the event. In 1868, General John A. Logan of the Grand Army of the Republic called for a "Decoration Day", which was widely celebrated. By 1890, every Northern state had adopted it as a holiday. The World Wars turned it into a generalized day of remembrance instead of just for the Civil War. In 1971, Congress standardized the holiday as "Memorial Day" and changed its observance to the last Monday in May.