Memorial Day is a holiday in the United States that commerates the men and women who gave their lives to their country while serving in the U.S. military. The holiday was first observed in honor of the Union soldiers of the American Civil War. After World War I, the holiday was expanded to include the casualties of any U.S. war or military action. The holiday is observed on the last Monday of the month of May. The holiday was formerly known as Decoration Day.