In 1884 the United Brotherhood of Carpenters introduced a resolution on the convention floor of the Federation of Organized Trades and Labor Unions. It called for a general strike to begin on May 1, 1886, for gaining the eight-hour workday as a standard for all workers in the United States. Most workers toiled for better than 12 to 16 hours on average, seven days a week, with no overtime, insurance or workers' compensation protections.
The resolution was adopted unanimously. [How's that for organized labor, setting a date for a strike two years in advance!] Struggles between labor and the police, National Guard, and the Pinkertons were violent in those times, and it was not uncommon for deaths to occur during clashes.
The newspapers and industrialists were instilling fear that May 1st, 1886 would be and inssurection, as the Paris Commune of 1871, that strikers were as foreign agents. As the day approached, more and more unions and workers were pledging to strike.
On May 1st, 1886, a saturday, but normally a workday, more than 350,000 workers walked off their jobs. The day is described in Labor's Untold Story, by Richard Boyer and Herbert Morais, thus: "May 1 was a beautiful day in Chicago. (...) crowds of workers, laughing, chatting, joking, and dressed in their best clothes, accompanied by their wives and children, were assembling for a parade on Michigan Avenue. (...) Off the main route of the parade and on streets adjacent to it, companies of armed police and special officers were gathering ready to enforce "law and order." On strategic rooftops police, Pinkertons, and militia officers were stationed with raifles and ohter paraphernalia of war. 1,350 members of the National Guard were mobilized and under arms, uniformed, equipped with Gatling guns." Among the speakers were Albert Parsons, a member of both the Knights of Labor and the Chicago Central Labor Union, and Albert Spies, the editor of the German workers' paper Die Arbeiter-Zeitung. All went peacefully, the militia demobilized, and police back to regular duty. But the strike continued.
On Monday violence errupted. Again, from Labor's Untold Story: "The police, exasperated by the futility of May I after such high expectations, gained some relief by clubbing the locked-out emplyees of the McCormick Harvester Works, as they ruched in 300 scabs. At closing time a great crowd of the lowcked-out employees were waiting the exit of the scabs when police suddenly charged them with drawn revolvers. The retreated when police, according to a witness, 'opened fire into their backs. Boys and men were killed as they ran.'" Six workers were killed.
The outraged leaders of the Chicago eight-hour movement organized a protest meeting in Haymarket Square the next day, May 4th. 180 police, came from the nearby station, in regular military formation. The crowd began to run. As the Police Captain Ward was demanding the meeting disperse, a bomb was thrown into a squad of police, killing eight. Police reacted by firing in every direction, and clubbing and trampling as they attempted to gain control. More than 200 of the participants were wounded when panicking police opened fire into the crowd.
In the following days, "the police, urged by the press and pulpit, by the great and the near great demainding immediate revenge, went wild, packing Chicago's jails (...) raiding the headquarters and offices of tradeunions and other working class organizations.
Among the arrested and indicted were Spies, Michael Schwab, Fielden, Parsons, Adolph Fischer, George Engel, Louis Lingg and Oscar Neebe. All were charged with conspiracy to murder, despite the fact that only three had been present at the Haymarket meeting. During the trial in June 1886, the state could not provide evidence that any of the men had knowledge of the bomb or that they had incited or participated in the violence.
As a result of the trial, all but one of the men received death sentences (Neebe received 15 years). Despite international outcry, Spies, Parsons, Fischer, and Engel were hanged on November 11, 1887; Lingg escaped by committing suicide.
In 1888, the American Federation of Labor voted to continue the eight-hour movement, fixing May 1st 1890 as the time for actions. The interactional labor movement, supported the eight-hour fight and designated May I, 1890 an interantional day of struggle.

Does this day still have meaning today? While in many developed countried there is an eight-hour day, in many this is still a goal. Many workers, even in developed coutries, find the need to world 12-hour days (or longer) to make ends meet. There are many violations of workers rights, and many struggles for gaining rights. Workers and union organizers still face violence everywehere.
Colombia is a nightmare in this regard. Thousands of trade-unionists have been assasinated since 1986. Three of every five unionists killed in the world are Colombian. The government and industrialists make use of the military and paramilitaries to specifically target union organizers. A worldwide campaign against Coca-Cola is based on significant proof of its complicity in murders. Ray Rogers, the president of Corporate Campaign, and a Coca-Cola shareholder brought these issues to the floor of a Coca-Cola share-holders meeting this April. Despite his right to talk, he has dragged away by six plainclothes Wilmington Police officers.
The class struggle continues. Perhaps the nature of class has changed since the 1880s, but as Michael Zweig in The working Class Majority points wout, one still has grupos of poeple with different degrees of power to determine and control the process in work and society.
The fact that the day is celebrated by millions worldwide (ironically, less so in the USA) is a testament to its continued significance. Millions of workers worldwide marched from Thailand to Ghana, South Africa to Russia, Germany to Canada, both Koreas to Cuba, Britain to Australia. Some of the events were marked by violence (such as Germany).