Why is Memorial Day on the last Monday in May?

Memorial Day (originally called Decoration Day) was one of the holidays included in the Uniform Monday Holiday Act passed by Congress, signed into law on June 28, 1968 and going into effect on January 1, 1971. This Act gave federal employees several three-day weekends during the year. Prior to that, Memorial Day was observed on May 30 no matter what day of the week it was.