Countries that observe Workers' Day/Labour Day on May 1st
This holiday remembers the 1886 Haymarket Affair, and the struggle for an eight-hour work day. src: Wikipedia and some extra research for ambiguous cases.
Countries which have a holiday for other reasons are not included: Finland, Ireland, Estonia, Kazakhstan
Two countries which had a holiday for other reasons that unofficially became Labour Day are included
Countries that move the holiday to the next Monday or something like that are included.