The Haymarket affair (also known as the Haymarket massacre or Haymarket riot)
was the aftermath of a bombing that took place at what began as a peaceful labor
rally on Tuesday May 4, 1886, at Haymarket Square in Chicago. Workers were
striking for an eight-hour work day when an unknown person threw a dynamite
bomb at police as they tried to disperse the demonstration. The bomb and
resulting gunfire left seven police officers and at least four civilians dead,
and many others wounded. The confrontation is generally considered
significant as the origin of international May Day labor observances.