Formerly known as Decoration Day, this holiday commemorates U.S. men and women who have died in military service to their country. It began first to honor Union soldiers who died during the American Civil War. After World War I, it expanded to include those who died in any war or military action.
The alternative name of "Memorial Day" was first used in 1882, but did not become more common until after World War II, and was not declared the official name by Federal law until 1967.
http://en.wikipedia.org/wiki/Memorial_Day