The Haymarket affair began as a peaceful rally held on May 4, 1886, in Haymarket Square, Chicago, in support of workers striking for an eight-hour workday. The protest drew various labor activists, including anarchists and socialists, who were advocating for workers' rights. Tensions between labor organizers and the police were already high due to earlier protests and violent encounters, with police cracking down on labor demonstrations. When police attempted to break up the gathering after it had begun to disperse, a bomb was thrown into the crowd, killing one officer instantly and injuring several others.

Following the bombing, panic erupted, and police began firing into the crowd. This ensuing gunfire resulted in the deaths of seven additional police officers and four civilians. The chaos and violence of the confrontation shocked the nation and led to widespread media coverage, painting labor activists in a negative light. The aftermath saw a significant crackdown on labor movements, with many leaders arrested and tried for conspiracy, even if they had not directly participated in violence.

The Haymarket affair became a pivotal moment for the labor movement in the United States. It caused a national backlash against labor organizations and contributed to a rising fear of anarchism during the late 19th century. The trial that followed featured heavily biased proceedings, leading to the execution and imprisonment of several prominent labor leaders. This event would forever be remembered as a symbol of the struggle for workers' rights in America, influencing labor laws and the eventual establishment of May Day as a celebration of labor.