The industrialization of Western economies during the XIXth century lowered working conditions of the middle class and gave rise to the proletariat. This working class only skilled in manpower and labor, could barely afford to house and feed themselves, even after working 10 hour days.
Beginning in the 1880's, numerous social movements were held with the goal of setting 8-hour workdays (the workday was 10 hours a day at the time). Strikes were the main mode of action, used to pressure employers.
In 1886 in the United States, a strike is called throughout the country, led particularly by anarchists. They decide on May 1st because it marks the new fiscal year and contract renewal date of American enterprises.
The U.S. strike of 1886 had an immense following with 340 000 members. In Chicago, a significantly industrial city, protesters are high in numbers and are severely reprimanded by the police. The following morning, protesters and anarchists hand out flyers to organize another protest the next day. On May 4th, protesters regroup at Haymarket Square. While the crowd begins to disperse, a bomb goes off near the police causing causalities and deaths on both sides. This, in turn, sets off violent backlash by both parties.
Subsequent to these events, authorities arrest ten anarchists notorious for their work. Amongst the ten, eight are accused guilty of the explosion. Five are sentenced to death, while the other three are sentenced to life in prison.
Seven years later in 1893, the governor of Illinois expunges the criminal record of those sentenced to death, and pardons the three sentenced to life in prison. He argues that the evidence presented in court is weak, and even suspects the Chicago police of organizing the attack, down to the explosion, as an excuse to justify the arrest of protesters.
This revelation provoked tremendous indignation in the world, and is considered one of the main reasons why May 1st is now trademarked as Labor Day.