“…May 1, ‘May Day,’ 1886, [is the day] when some 200,000 U. S. workmen engineered a nationwide strike for an eight-hour workday…
“This labor action wasn’t just any strike—it was part of what became known as the Haymarket Square’ On May 1 of that year, Chicago (along with other cities) was the site of a major union demonstration in support of the eight-hour workday. The Chicago protests were meant to be part of several days of action. On May 3, a strike at the McCormick Reaper plant in the city turned violent; the next day, a peaceful meeting at Haymarket Square became even more so….”
Read more, if you want, about the history of this day in the history of the labor movement in the United States and around the world.