"May Day," 1 May, is International Workers' Day, when many all over the world (with some notable exceptions) celebrate the successes of the labor movement.
On May Day, the world commemorates May, 1, 1886, when the eight-hour workday was supposed to come into effect in the United States.
The eight-hour workday had been a demand from workers and labor unions for decades. Some legislation had been passed, but was largely ineffective. At its Chicago convention in 1884, the Federation of Organized Trades and Labor Unions determined that "eight hours shall constitute a legal day's labour from and after May 1, 1886, and that we recommend to labour organisations throughout this jurisdiction that they so direct their laws as to conform to this resolution by the time named."
Two years later, manufacturers and factory owners would not concede. 80,000 people marched in protest in Chicago, and in the next few days 190,000 went on strike, while 350,000 marched in cities all over the United States. On May 3rd, a march in Chicago turned violent and deadly: the police shot at workers at the McCormick plant, killing four and wounding many. The next day, at a protest against the violence, a bomb exploded in Haymarket Square.
This was the signal for the police to round up hundreds of activists and their leaders. Though they had nothing to do with the bomb, five of them (Parsons, Engel, Lingg, Spies, Fisher) were sentenced to death by hanging in an unfair trial. Lingg was found dead in his cell. When the remaining hapless four were executed on "Black Friday", 11 November 1886, half a million Americans walked in protest. The pardon in 1893 came far too late. Most workers in the United States had to wait until 1938 for their eight-hour workday.
The American workers' struggle was paralleled in countries all over the world. May 1st was declared their day, the International Workers' Day or International Labor Day, by the Second Marxist National (the conference of socialist and labor parties) in 1889. In many countries it is an official holiday when people commemorate the struggle for workers' rights and protest all kinds of ills of society in meetings, protest rallies and speeches.
Ironically, and unfortunately perhaps, in the United States the commemoration was moved to Labor Day (the first Monday of September), which is more a day of leisure than of labor, a day of picnics, not protests, a day of family, not society.
For many others, May Day is a salute to Spring. More about that in
Labor Day/Labour Day in the US and Canada