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 at Haymarket Square in Chicago.
A sympathetic engraving by Walter Crane of the executed “Anarchists of Chicago” after the Haymarket affair. The Haymarket affair is generally considered the most significant event for the origin of international May Day observances.
On May as police approach a labor rally in Chicago, a bomb is thrown. The resultant trial of eight activists brought worldwide attention to the labor movement, and initiated the tradition of May Day labor rallies in many cities.