May 1st was declared a workers holiday by the International Working Men's Association (First International) in Paris in 1889.
(It was to commemorate the Haymarket Martyrs of 1886: 8 anarchists who were wrongly accused of throwing a bomb at police. 4 of them; Albert Parsons, August Spies, Adolf Fischer and George Engel were hanged, Louis Lingg committed suicide in prison and in 1893, John Peter Altgeld, pardoned the three surviving men; Samuel Fielden, Oscar Neebe and Michael Schwab. A bit too late though.)