International Workers' Day, also known as Labor Day or May Day, is an annual celebration of the working class celebrated on this day, established by the 2nd International to commemorate the 8 hour day strike that caused the Haymarket Affair.

The Haymarket Affair began in Chicago, Illinois with a general strike to demand an eight-hour working day. On May 4th, the police attempted to disperse a public gathering in support of the strike and an unidentified person threw a bomb into the police line, killing several officers.

The police responded by firing on the crowd of protesters. The ensuing violence led to the deaths of seven police officers, four to eight civilians, and wounded approximately one hundred people on either side.

In a hysterical crackdown, hundreds of labor leaders and sympathizers were detained, many being guilty of nothing more than having political sympathies for labor. Four of them, George Engel, Albert Parsons, August Spies, and Adolph Fischer, were executed by hanging after conviction in a trial widely perceived as a miscarriage of justice. The following day, in Milwaukee, Wisconsin, state militia fired on a crowd of strikers killing seven, including a schoolboy and a man feeding chickens in his yard.

May Day has been frequently chosen by labor activists across the world as a day to initiate strikes, boycotts, and other forms of protest.