May Day originated with the United States labor movement in the late 19th Century. On May 1, 1886, unions across the country went on strike, demanding that the standard workday be shortened to eight hours. The organizers of these strikes included socialists, anarchists, and others in organized labor movements. Rioting in Chicago’s Haymarket Square on May 4th including a bomb thrown by an anarchist led to the deaths of a dozen people (including several police officers) and the injury of over 100 more.
The protests were not immediately successful, but they proved effective down the line, as eight-hour work days eventually did become the norm. Labor leaders, socialists, and anarchists around the world took the American strikes and their fallout as a rallying point, choosing May Day as a day for demonstrations, parades, and speeches. It was a major state holiday in the Soviet Union and other communist countries.
Labor Day is still celebrated on May 1 in countries around the world, and it is still often a day for protests and rallies. In recent years, these have often been targeted against globalization.