On what date did the Haymarket affair take place in Chicago?

The Haymarket affair took place in Chicago on May 4, 1886.

A labor rally was held in Haymarket Square to protest police violence during a strike at the McCormick Reaper Works. The meeting was initially peaceful, but police moved to disperse the crowd. Someone threw a bomb, and police and demonstrators exchanged gunfire. The exact identity of the bomb thrower was never established.

Several anarchists were convicted in a controversial trial, and the event became an international symbol of the labor movement. The affair also influenced the choice of May 1 for International Workers’ Day, although the event itself occurred on May 4.

Haymarket is sometimes called the Haymarket massacre, but “Haymarket affair” is the common historical article title. The incident occurred in Chicago, not at the later Pullman company town.