Today is Labor Day in the United States and Canada. Even though the May Day holiday observed in the rest of the world actually has its origins in America, President Grover Cleveland chose the September date when Labor Day became a Federal holiday in 1894. The establishment of a workers’ day was a conciliatory gesture to re-establish good relations with the American union movement after the Pullman Strike in that year, during which US troops had opened fire on strikers, killing 13.
But Cleveland was unwilling to adopt the new international May Day holiday because it commemorated an affair which showed the political establishment in an even worse light: the 1886 Haymarket Massacre and subsequent trial and executions.
1st May 1886 was the target date set by the American and Canadian Federation of Organized Trades and Labor Unions for achievement of a maximum eight-hour working day.
They didn’t actually expect to be successful, and were planning mass demonstrations and strikes on and after this date. Trouble broke out and four strikers were shot dead by police in Chicago on 3rd May. A protest demonstration the next evening at Haymarket Square turned into a bloody riot. A thrown bomb killed one police officer, and six others were shot dead, along with at least four civilians in the crowd, and many were injured.
The vast majority of the protesters had been peaceful, but there were certainly some anarchists among them who were prepared to use violence; and the police and Pinkerton’s security men opened fire indiscriminately. (So many officers were wounded by their own side that it’s not even known if there was any firing at all from the crowd.)
The (peaceful) organisers of the demonstration were arrested, tried for conspiracy and executed. The bomber was never identified. (The prosecution admitted that none of the defendants in court was thought to have thrown the bomb.)
The first international May Day demonstrations were in 1890, partly to commemorate the victims of the Haymarket riot, but also “for the legal establishment of the 8-hour day, for the class demands of the proletariat, and for universal peace.”