Holidays on May 1st
May Day which has its traditional origins in the Celtic holiday of Beltane and the Germanic holiday of Walpurgis Night.
International Worker's Day [aka Labo(u)r Day] which has its origins in Chicago's Haymarket Affair. [Chicago Historical Society's Haymarket Affair Digital Collection]
Loyalty Day - a holiday created in America in the 1950s to 'counter-balance' International Worker's Day.
MayDay - "an initiative to protect cultural heritage from disasters." (hat/tip to Ancestories, as I was unfamiliar with this initiative.)
Note: the distress call "Mayday!" actually derives from the French word m'aider - meaning "come help me."