Memorial Day is a United States federal holiday that is observed on the last Monday of May. It was 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 was expanded to include those who died in any war or military action. One of the longest standing traditions is the running of the Indianapolis 500, which has been held in conjunction with Memorial Day since 1911.