This holiday is most commonly associated as a commemoration of the achievements of the labor movement.
The 1 May date is used because in 1884 the American Federation of Organized Trades and Labor Unions demanded an eight-hour workday, to come in effect as of 1 May 1886. This resulted in the general strike and the Haymarket Riot of 1886, but eventually also in the official sanction of the eight-hour workday.
May Day is celebrated as Labour Day in most countries around the world. In the United Kingdom the bank holiday isn't fixed at May 1st, but instead is observed on the first Monday of May.
In the 20th century, the holiday received the official endorsement of the Soviet Union, and it is also celebrated as the Day of the International Solidarity of Workers, especially in some Communist states. Celebrations in communist countries during the Cold War era often consisted of large military parades with the latest weaponry being exhibited as well as shows of common people in support of the government.
Curiously (given the origin of the 1 May date), the United States celebrates Labor Day on the first Monday of September (1 May is Loyalty Day, a legal but not widely recognized holiday in the United States). There is some suggestion that the reason for this was to avoid the commemoration of riots that had occurred in 1886. The adoption of May Day by communists and socialists as their primary holiday have been as a another reason as they further inceased official resistance to May Day labor celebrations in America.
May Day has long been a focal point for demonstrations by various communist, socialist, and anarchist groups.
1 May was also a pagan holiday in many parts of Europe, Its roots as a holiday stretch back to the Gaelic Beltane. It was considered the last day of winter when the beginning of summer was celebrated.
During Roman times, 1 May was seen as a key period to celebrate fertility and the arrival of spring. The Roman festival of Flora, the goddess of flowers and the season of spring, was held between 28 April and 3 May.
Traditional English May Day rites and celebrations include Morris dancing, crowning a May Queen, and dancing around a Maypole; festivities which made it a popular seasonal celebration in medieval England.
Labour Day was established as an official holiday in 1933 after the NSDAP rose to power. It was supposed to symbolize the new found unity between the state and the working classes. Ironically, just one day later, on May 2 1933, all free unions were outlawed and destroyed. But since the holiday had been celebrated by German workers for many decades before the official state endorsement, the NSDAP attempt to appropriate it left no long-term resentment.
In Sweden May day is also a Christian celebration; the Mass of Saint Walburga or Walpurgis Night is celebrated on the evening of 30 April.
A key May Day celebration in Italy is the the annual Concerto del Primo Maggio which takes place in Rome and is attended by more than half a million people.