The latest possible date for U.S. Memorial Day is May 31.
Federal law schedules Memorial Day for the last Monday in May. If May 31 is a Monday, the holiday occurs on that date; otherwise it falls earlier. The rule means Memorial Day can occur on any date from May 25 through May 31, inclusive.
This calendar rule has applied since the federal Monday holiday schedule took effect in 1971. Before then, Memorial Day was commonly observed on May 30, a fixed date associated with the earlier Decoration Day tradition.
The date is unrelated to the date of the first national Decoration Day observance in 1868. It also differs from Veterans Day, which retains the fixed date November 11. These separate rules are a common source of confusion when comparing U.S. military-related holidays.