"The Haymarket affair (also known as the Haymarket massacre or Haymarket riot) refers to 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. An unknown person threw a dynamite bomb at 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. The evidence was that one of the defendants may have built the bomb, but none of those on trial had thrown it. 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 rather than face the gallows. 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 on March 25, 1992, and a public sculpture was dedicated at the site in 2004. The Haymarket Martyrs' Monument in nearby Forest Park was designated a National Historic Landmark on February 18, 1997, and listed on the National Register of Historic Places."