The eight-hour day was not a gift from employers or the state but a conquest wrested from capital through decades of bitter struggle. As early as 1867, Marx observed in Capital that "the first fruit of the Civil War was the eight hours' agitation," and by 1886 the movement had reached a crescendo. On May 1, 1886, more than 300,000 workers nationwide—40,000 in Chicago alone—walked off the job under the slogan

"Eight hours for work, eight hours for sleep, eight hours for what we will!"

Chicago was the epicenter, an overwhelmingly immigrant, international workforce confronting the "robber barons".  Field, Pullman, Armour, McCormick, men whose obscene fortunes rested on the most brutal exploitation.

The struggle was met with police terror, culminating on May 4, when police attacked a peaceful rally at Haymarket Square called in response to the killing of workers at the McCormick Reaper Works.

A bomb exploded in the ensuing melee, and the authorities seized the opportunity to railroad the movement's leaders.  Albert Parsons, August Spies, Adolph Fischer, and George Engel through a sham trial. They were hanged on November 11, 1887, framed for a crime they did not commit.

It was out of this that May Day was born. In 1889, the Second International, meeting in Paris  resolved to hold "a great international demonstration" each May 1 to commemorate the Haymarket Martyrs and the fight for the eight-hour day. Demonstrations swept Europe and North America in 1890, and Russian and Chinese workers took up the date in 1891 and 1927.

The American ruling class, terrified of the revolutionary character of the observance, moved to dilute the day. 

The September Labor Day holiday was instituted precisely to obscure May Day and to substitute a patriotic, nationally bounded observance for a day of international working-class solidarity.

At work sending this on my phone on a 10 minute break.  To see if it works.  The CSS needs work but I expect this message will post.