Memorial Day is a United States federal holiday celebrated on the last Monday of May commemorating men and women who died while serving in the United States Armed Forces.
Initially (officially since 1868 - proclaimed by Genearal John Logan) Memorial Day was a holiday in memory of soliders who died in the Civil War. The feast was supposed to be a step towards national reconciliation. The original name (Decoration Day) comes from placing flowers on the graves of the soliders.
Memorial Day was celebrated differently in different states. Until 1890 it was celebrated in all the states of the North. After World War I the nature of the holiday changed to honor the memory of all Americans who died in any war - not only Civil.