I was going to write something up about this myself, but time conspired against me. Instead I'll re-produce part of an article written by Terry McPartlan, published on Marxist.com:
May Day, international workers' day. On every continent the advanced layers of workers and youth celebrate internationalist ideas and the struggle of the Labour movement. No surprise that the Tories in Britain tried to eradicate the holiday. No surprise either that workers' demonstrations are the focus of state repression around the world.
But where does the modern May Day come from, who started it and why?
May Day sprang from the struggle of the American working class. In 1884 the American Federation of Labor adopted the following demand:
"Resolved by the Federation of Organized Trades and Labor Unions of the United States and Canada, that eight hours shall constitute a legal day's labor from May 1st, 1886, and that we recommend to labor organizations throughout their jurisdiction that they so direct their laws as to conform to this resolution by the time named."
This slogan became the focus of a whole series of movements around the eight- hour day.
On May 1st 1886, Albert Parsons of the Chicago Knights of Labor led 80,000 workers on a demonstration through Chicago in support of the campaign for an 8-hour day. They weren't alone. Within a few days 350,000 workers took strike action across the country involving 1200 factories. 70,000 struck in Chicago alone.
August Spies, editor of the ArbeiterZeitung (Workers Newspaper), spoke at a meeting of 6,000 workers on May 3rd. Following the meeting many of the participants moved down the street to harass scabs entering the McCormick plant. The police arrived, opened fire, and killed four people, wounding many more.
If you want to know how it finishes, including how the US authorities sentenced 7 people to death, read the rest here.
Not mentioned in the article however is the Second Communist International later decided to hold mass demonstrations, internationally, on the anniversary of the events in Chicago, it was so successful it became an annual event from 1891 onwards.