Memorial Day is a United States federal holiday observed on the last Monday of May (on May 25 in 2009). Formerly known as Decoration Day, it commemorates U.S. men and women who died while in military service to their country. First enacted to honor Union soldiers of the American Civil War (it is celebrated near the day of reunification after the civil war), it was expanded after World War I to include American casualties of any war or military action.
The following article explains the history of Memorial Day and how it is currently celebrated in the U.S.:
http://www.america.gov/st/washfile-english/2006/May/20060524125235jmnamdeirf7.503909e-02.html