Memorial Day is a United States federal holiday observed on the last Monday of May. Formerly known as Decoration Day, it commemorates U.S. Service Members who died while in the military service. First enacted by formerly enslaved African-Americans  to honor Union soldiers of the American Civil War – it was extended after World War I to honor Americans who have died in all wars.
Memorial Day often marks the start of the summer vacation season, and Labor Day its end.
Begun as a ritual of remembrance and reconciliation after the Civil War, by the early 20th century, Memorial Day was an occasion for more general expressions of memory, as ordinary people visited the graves of their deceased relatives, whether they had served in the military or not. It also became a long weekend increasingly devoted to shopping, family get-togethers, fireworks, trips to the beach, and national media events such as the Indianapolis 500 auto race, held since 1911 on the Sunday of Memorial Day weekend.
Info from: Wikipedia