May Day, as an international workers' celebration, dates back to what 1886 event?

The Haymarket affair (also known as the Haymarket massacre or Haymarket riot) was 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 and in reaction to the killing of several workers the previous day by the police. 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.

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 in 1992, and a public sculpture was dedicated there in 2004. In addition, the Haymarket Martyrs' Monument at the defendants' burial site in nearby Forest Park was designated a National Historic Landmark in 1997.

The first international May Day was a spectacular success. The front page of the New York World on May 2, 1890, was devoted to coverage of the event. The Times of London listed two dozen European cities in which demonstrations had taken place, noting there had been rallies in Cuba, Peru and Chile. Commemoration of May Day became an annual event the following year.