The date is promoted by by socialist and communist political parties to commemorate the Haymarket affair, which occurred in Chicago on 4 May 1886.
The Federation of Organized Trades and Labor Unions (which would later become the American Federation of Labor, or AFL) held a convention in Chicago in 1884 and proclaimed “eight hours shall constitute a legal day’s labor from and after May 1, 1886.”
The following year the Knights of Labor – then America’s largest labor organization – backed the proclamation and both groups encouraged workers to strike and demonstrate in favor of the demand.
On May 1, 1886, more than 300,000 workers (40,000 in Chicago alone) from 13,000 business walked out of their jobs across the country. In the following days, more workers joined and the number of strikers grew.
On May 3, Chicago police and workers clashed at the McCormick Reaper Works. The next day a rally was held at Haymarket Square to protest the killing and wounding of several workers by the police.
When the speaker, August Spies, was winding down, a group of officers arrived to disperse the crowd. As the police advanced, a bomb was thrown into their ranks. Chaos ensued, and at least seven police officers and eight civilians died as a result of the violence that day.
A few years after the Haymarket Riot and subsequent trials shocked the world, a newly formed coalition of socialist and labor parties in Europe called for a demonstration to honor the “Haymarket Martyrs.” In 1890, over 300,000 people protested at a May Day rally in London.
Subsequently, the Russian bolsheviks celebrated May Day with fervor.