From here: History:
Our modern celebration of Mayday as a working class holiday evolved from the struggle for the eight hour day in 1886. May 1, 1886 saw national strikes in the United States and Canada for an eight hour day called by the Knights of Labour. In Chicago police attacked striking workers killing six.
The next day at a demonstration in Haymarket Square to protest the police brutality a bomb exploded in the middle of a crowd of police killing eight of them. The police arrested eight anarchist trade unionists claiming they threw the bombs. To this day the subject is still one of controversy. The question remains whether the bomb was thrown by the workers at the police or whether one of the police’s own agent provocateurs dropped it in their haste to retreat from charging workers.
In what was to become one of the most infamous show trials in America in the 19th century, but certainly not to be the last of such trials against radical workers, the State of Illinois tried the anarchist workingmen for fighting for their rights as much as being the actual bomb throwers. Whether the anarchist workers were guilty or innocent was irrelevant. They were agitators, fomenting revolution and stirring up the working class, and they had to be taught a lesson.
Albert Parsons, August Spies, George Engle and Adolph Fischer were found guilty and executed by the State of Illinois.
In Paris in 1889 the International Working Men’s Association (the First International) declared May 1st an international working class holiday in commemoration of the Haymarket Martyrs. The red flag became the symbol of the blood of working class martyrs in their battle for workers rights.
Mayday, which had been banned for being a holiday of the common people, had been reclaimed once again for the common people.