130 years ago, workers in Chicago assembled in the United States (8 hours of work day), but a peaceful demonstration turned into a violent incident. A bomb was thrown to the police, and the police fired. At least 7 police and 4 civilians were killed in the chaos.
3 years after the event, the world workers' movement organization "Socialist International" (also known as the second International) held a conference in Paris. The meeting decided to hold similar meetings in May 1st every year. At the same time, to commemorate the tragedy of the Chicago workers' movement and the hay market, the General Assembly passed the resolution of the May day case. The year May 1st is designated as the international labor day.