A holiday is day set aside by a nation or culture (in some cases, multiple nations and cultures) typically for celebration but sometimes for some other kind of special culture-wide (or national) observation or activity.
Based on the English words holy and day, holidays originally represented special days of the Christian Church calendar. The word has evolved in general usage to mean any special day, or even non-special day on which school and/or offices are closed such as Sunday.
The US Congress changed the observance of Memorial Day, Veteran's Day, and Washington's Birthday from fixed dates to certain Monday's in 1968 (effective 1971). Several states had passed similar laws earlier.