May 1 1886 saw 340,000 workers striking all over the United States demanding an 8-hour day.
In Chicago alone, 80,000 came out and there a number of anarchist militants agitated inside the movement.
The following Monday, the police fired on strikers at the McCormick Harvester works and six workers were killed. The next day a protest meeting at Haymarket Square was broken up by the police. In the following confusion, a bomb was thrown at the police, killing one outright and fatally wounding 7 others.
Evidence came to light later that the bomb had been thrown by a police agent. The bosses, however, used this incident to victimise leading working class militants and attempt to break the movement.
After a farcical trial, with a jury made up of businessmen, their clerks, and a relative of a dead policeman, four anarchists were hanged (Albert Parsons, August Spies, George Engel and Adolph Fishcer) some of whom hadn't even been at the protest meeting! Another anarchist (Louis Lingg) escaped the noose by taking his own life the day before execution.
At the funeral, an estimated 500,000 people lined the route and up to 25,000 saw the burial.
Seven years after the execution, an inquiry found those executed innocent of all charges and 3 anarchists serving life sentences were released. Workers organisations from then on made May 1st a celebration of the Haymarket Martyrs.