Chicago gave its name to the 1886 labor protest remembered as the Haymarket affair.
The event began as a workers’ meeting in Haymarket Square on May 4, 1886, during a nationwide campaign for an eight-hour working day. The meeting followed violence at McCormick Reaper Works, where police had fired on striking workers the previous day.
A bomb was thrown as police moved to disperse the meeting, and gunfire followed. Several police officers and civilians died, and the subsequent trial of labor activists became internationally controversial. The affair became a powerful symbol for labor movements around the world.
May 1 was already associated with the eight-hour movement, but the Haymarket affair helped strengthen its symbolic importance. The event is sometimes incorrectly dated to May 1; the Haymarket meeting itself took place on May 4.