Our modern celebration of May Day as a working class holiday developed from the US workers struggle for the eight hour day in 1886. The working class movement in the USA began campaigning for an eight hour day in the 1860s, following the Civil War. The historic strike of May 1st, 1886 was a culmination of a concerted struggle. Chicago was the major industrial centre of the USA. Police attacked striking workers from the McCormack Harvester Co., killing six.
On May 4th 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 the19th 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 FirstInternational) 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.