Three years after the Civil War ended, on May 5, 1868, the head of an organization of Union veterans — the Grand Army of the Republic (GAR) — established Decoration Day as a time for the nation to decorate the graves of the war dead with flowers. Maj. Gen. John A. Logan declared that Decoration Day should be observed on May 30. It is believed that date was chosen because flowers would be in bloom all over the country.
It was not until after World War I, however, that the day was expanded to honor those who have died in all American wars. In 1971, Memorial Day was declared a national holiday by an act of Congress, though it is still often called Decoration Day. It was then also placed on the last Monday in May.
Source: U.S. Department of Veterans Affairs