"May 1st became a commemoration by the socialist Second International for the people involved in the 1886 Haymarket affair.
The Haymarket affair occurred during the course of a three-day general strike in Chicago, Illinois, United States that involved common laborers, artisans, merchants, and immigrants. Following an incident in which police opened fire and killed four strikers at the McCormick Harvesting Machine Co. plant, a rally was called for the following day at Haymarket Square. Towards the end of the rally, as police moved in to disperse the event and opened fire on the unarmed crowd on the plea that an unknown assailant threw a bomb into the crowd of police. The bomb and resulting riot left at least a dozen people dead, including one policeman" .
In those times the "standard" daily working hours were from 12 to 16; from Monday to Saturday; children included. Most of the struggle was due to the union's request of a working day of 8 hours from Monday to Saturday. Several years later came the week of 40 hours and finally in the 1930's arrived the one week a year paid vacations. It was a long fight with a lot of casualties among the organized working women and men.
Today, May Day has become an international celebration of the social and economic achievements of the labour movement.