The Haymarket affair was the aftermath of a bombing that took place at a labor demonstration on Tuesday May 4, 1886, at Haymarket Square in Chicago. It began as a peaceful rally in support of workers striking for an eight-hour day and in reaction to the killing of several workers the previous day by the police. An unknown person threw a dynamite bomb at Chicago police as they acted to disperse the public meeting. The bomb blast and ensuing gunfire resulted in the deaths of seven police officers and at least four civilians; scores of others were wounded.
In the internationally publicized legal proceedings that followed, eight anarchists were convicted of conspiracy. Seven were sentenced to death and one to a term of 15 years in prison. The death sentences of two of the defendants were commuted by Illinois governor Richard J. Oglesby to terms of life in prison, and another committed suicide in jail. The other four were hanged on November 11, 1887. In 1893, Illinois' new governor John Peter Altgeld pardoned the remaining defendants and criticized the trial.
The Haymarket affair is generally considered significant as the origin of international May Day observances for workers. The site of the incident was designated a Chicago Landmark in 1992, and a public sculpture was dedicated there in 2004. In addition, the Haymarket Martyrs' Monument at the defendants' burial site in nearby Forest Park was designated a National Historic Landmark in 1997.