Whether you celebrate it like this:
International Workers' Day is the commemoration of the 1886 Haymarket Massacre in Chicago, when, after an unknown person threw a dynamite bomb at police as they dispersed a public meeting, Chicago police fired on workers during a general strike for the eight hour workday, killing several demonstrators and resulting in the deaths of several police officers, largely from friendly fire. In 1889, the first congress of the Second International, meeting in Paris for the centennial of the French Revolution and the Exposition Universelle, following a proposal by Raymond Lavigne, called for international demonstrations on the 1890 anniversary of the Chicago protests. May Day was formally recognized as an annual event at the International's second congress in 1891.
Subsequently, the May Day Riots of 1894 occured. In 1904, the International Socialist Conference meeting in Amsterdam called on "all Social Democratic Party organizations and trade unions of all countries to demonstrate energetically on May First for the legal establishment of the 8-hour day, for the class demands of the proletariat, and for universal peace." The congress made it "mandatory upon the proletarian organizations of all countries to stop work on May 1, wherever it is possible without injury to the workers."
In many countries, the working classes sought to make May Day an official holiday, and their efforts largely succeeded. May Day has long been a focal point for demonstrations by various socialist, communist and anarchist groups. In some circles, bonfires are lit in commemoration of the Haymarket martyrs, usually at dawn. May Day has been an important official holiday in Communist countries such as the People's Republic of China, Cuba and the former Soviet Union. May Day celebrations typically feature elaborate popular and military parades in these countries.
In the United States and Canada, however, the official holiday for workers is Labor Day in September. This day was promoted by the Central Labor Union and the Knights of Labor, who organized the first parade in New York City. After the Haymarket Square riot in May, 1886, US President Grover Cleveland feared that commemorating Labor Day on May 1 could become an opportunity to commemorate the riots. Thus he moved in 1887 to support the Labor Day that the Knights supported.
In 1955, the Roman Catholic Church dedicated May 1 to "Saint Joseph The Worker". The Catholic Church considers Saint Joseph the patron saint of (among others) workers, craftsmen, immigrants and "people fighting communism".
Right-wing governments have traditionally sought to repress the message behind International Workers' Day, with facist governments in Portugal, Italy, Germany and Spain abolishing the workers' holiday, and the Conservative party in the UK currently attempting to abolish the UK's annual May Day Bank Holiday.
Or like this
May Day is related to the Celtic festival of Beltane and the Germanic festival of Walpurgis Night. May Day falls exactly half of a year from November 1, another cross-quarter day which is also associated with various northern European pagan and neopagan festivals such as Samhain. May Day marks the end of the unfarmable winter half of the year in the Northern hemisphere, and it has traditionally been an occasion for popular and often raucous celebrations.
As Europe became Christianized the pagan holidays lost their religious character and either changed into popular secular celebrations, as with May Day, or were merged with or replaced by new Christian holidays as with Christmas, Easter, Pentecost and All Saint's Day. In the twentieth century, many neopagans began reconstructing the old traditions and celebrating May Day as a pagan religious festival again.
The earliest May Day celebrations appeared in pre-Christian times, with the festival of Flora, the Roman Goddess of flowers, and the Walpurgis Night celebrations of the Germanic countries. It is also associated with the Gaelic Beltane. Many pagan celebrations were abandoned or Christianized during the process of conversion in Europe. A more secular version of May Day continues to be observed in Europe and America. In this form, May Day may be best known for its tradition of dancing the Maypole and crowning of the Queen of the May. Various Neopagan groups celebrate reconstructed (to varying degrees) versions of these customs on May the 1st. The day was a traditional summer holiday in many pre-Christian European pagan cultures. While February 1 was the first day of Spring, May 1 was the first day of summer; hence, the summer solstice on June 25 (now June 21) was Midsummer. In the Roman Catholic tradition, May is observed as Mary's month, and in these circles May Day is usually a celebration of the Blessed Virgin Mary. In this connection, in works of art, school skits, and so forth, Mary's head will often be adorned with flowers in a May crowning. Fading in popularity since the late 20th century is the giving of "May baskets," small baskets of sweets and/or flowers, usually left anonymously on neighbours' doorsteps.