Three years after the Civil War ended, on May 5, 1868, the head of the organization of Union Veterans – the Grand Army of the Republic (GAR) – established Decoration Day as a time to decorate the graves of the war dead with flowers. Major General 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.
The preferred name for the holiday gradually changed from “Decoration Day” to “Memorial Day”, which was first used in 1882. The usage of the name declared the official name by Federal law until 1967. On June 28,1968, Congress passed the Uniform Monday Holiday Act, which moved several holidays, including Memorial Day, from their traditional dates to a specified Monday in order to create a convenient three-day weekend. The change moved Memorial Day from its traditional May 30th date to the last Monday in May. The law took effect at the federal level in 1971. (source Wikipedia)