In many countries around the world, today is a national holiday, commemorating something that happened in Chicago, on this day in 1886.

Industrial workers had assembled to protest in support of a radical change, something we all take for granted now: the eight-hour work day.
Led by the Chicago Knights of Labor, the city's workers began a rapidly spreading series of strikes. Within days, 350,000 workers at factories across the country had stopped working in solidarity. But unfortunately, things in Chicago got out of hand.
On May 3, police opened fire on a crowd of 6,000 demonstrators outside the McCormick Reaper Works Factory, killing four and wounding many more. The next day, 2,500 people showed up at a memorial-cum-protest rally. As the rally drew to a close, 180 police showed up. And somebody threw a bomb.
In the chaos that ensued, eight police died (one from the bomb, seven from friendly fire) along with four demonstrators.
A massive crackdown on leftists and unionists followed. Eight "radicals" (only two of whom were present at the time of the bombing) were tried and sentenced to death. Years later, they were officially pardoned, and the three survivors were allowed to go free. But the labor movement never fully recovered, and May Day is celebrated in almost every country in the world... except the United States.
In 1992, Mayor Richard M. Daley dedicated a plaque at the site of the riots, which reads:
"A decade of strife between labor and industry culminated here in a confrontation that resulted in the tragic death of both workers and policemen. On May 4, 1886, spectators at a labor rally had gathered around the mouth of Crane's Alley. A contingent of police approaching on DesPlaines Street were met by a bomb thrown from just south of the alley. The resultant trial of eight activists gained worldwide attention for the labor movement, and initiated the tradition of "May Day" labor rallies in many cities."
So in their honor, I think it's only fitting that you stop what you're doing, stand up wherever you are, and sing at the top of your lungs. This is a song the Haymarket rioters would have known well, written in honor of the Paris Commune in spring, 1871... the first time industrial workers took governmental power into their own hands.
And here's food for thought -- when the pro-democracy students faced the tanks of the People's Republic in Tianenmen Square, this is the song they were singing.
Words and music below:
Arise ye workers from your slumbers
Arise ye prisoners of want
For reason in revolt now thunders
And at last ends the age of cant.
Away with all your superstitions
Servile masses arise, arise!
We'll change henceforth the old tradition
And spurn the dust to win the prize.
So comrades, come rally
And the last fight let us face
The Internationale unites the human race.
So comrades, come rally
And the last fight let us face
The Internationale unites the human race.No more deluded by reaction
On tyrants only we'll make war
The soldiers too will take strike action
They'll break ranks and fight no more
And if those cannibals keep trying
To sacrifice us to their pride
They soon shall hear the bullets flying
We'll shoot the generals on our own side.No saviour from on high delivers
No faith have we in prince or peer
Our own right hand the chains must shiver
Chains of hatred, greed and fear
E'er the thieves will out with their booty
And give to all a happier lot.
Each at the forge must do their duty
And we'll strike while the iron is hot.
You can read more about the Internationale here (and hear versions in Magyar, Zulu and Tagalog here).