Prior to 1971 ‘Memorial Day‘ (which was originally known as ‘Decoration Day‘) was always celebrated on ‘May 30’ — irrespective of what day of the week it was. The holiday goes back to the end of the Civil War.
In 1968 Congress, with the ‘Uniform Holidays Bill‘ aimed at creating 3-day weekends, mandated that ‘Memorial Day’ would be on the last Monday of May. As such it can fall anywhere from May 25 to May 31. This year will be the 7th time, since 1971, it has fallen on May 30.
May 26, May 28 and May 31 have already had 7 instances. Falls the least often on the earliest possible day, May 25.