Memorial Day (originally known as Decoration Day) is a U.S. federal holiday commemorating U.S. service members who die while serving in the U.S. armed forces.  It is celebrated on the last Monday in May. It was previously 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 US military. Many volunteers plant the American flag on the graves of service members in national cemeteries. Memorial Day is also considered the unofficial start of summer in the United States.
Many cities and people claim to be celebrating this event for the first time. In 1868, General John A. Logan of the Grand Army of the Republic called a “Commendation Day” which was widely celebrated. By 1890, all northern states had adopted it as a holiday. The World Wars made it a widespread day of remembrance, not just the Civil War. In 1971, Congress standardized the holiday as “Memorial Day” and changed it to the last Monday in May.
Two other days commemorate those who have served or are serving in the U.S. military: Armed Forces Day (which is celebrated in early May), an unofficial U.S. holiday honoring those currently serving in the military, and Veterans Day (November 11) . which honors those who have served in the United States military.
source:Memorial Day – Wikipedia