Until 1971, Memorial Day always occurred on May 30th.

This changed with a 1968 law known as the Uniform Monday Holiday Act which set George Washington's Birthday (later President's Day), Memorial Day, Labor Day, and Columbus Day to Mondays.

Previously, all of these holidays were set days that would float with the week. Between 1971 and 1978 Veterans Day was always the second Monday in November, but Congress decided to return it to November 11th starting in 1979 -- when they created Martin Luther King Day as the third Monday in January.