International Workers' Day, also called Labour Day in some countries and often referred to as May Day, is a celebration of labourers and the working classes that is promoted by the international labour movement and occurs every year on 1 May, or the first Monday in May.

Traditionally, 1 May is the date of the European spring festival of May Day. The International Workers Congress held in Paris in 1889 established the Second International for labor, socialist, and Marxist parties. It adopted a resolution for a "great international demonstration" in support of working-class demands for the eight-hour day. The date was chosen by the American Federation of Labor to commemorate a general strike in the United States, which had begun on 1 May 1886 and culminated in the Haymarket affair on 4 May. The demonstration subsequently became a yearly event. The 1904 Sixth Conference of the Second International, called on "all Social Democratic Party organisations and trade unions of all countries to demonstrate energetically on the First of May for the legal establishment of the eight-hour day, for the class demands of the proletariat, and for universal peace".

The 1st of May, or first Monday in May, is a national public holiday in many countries, in most cases known as "International Workers' Day" or a similar name. Some countries celebrate a Labour Day on other dates significant to them, such as the United States and Canada, which celebrate Labor Day on the first Monday of September. In 1955, the Catholic Church dedicated 1 May to "Saint Joseph the Worker". Replacing another feast to St. Joseph, this date was chosen by Pope Pius XII in 1955 as a counterpoint to the communist International Workers' Day celebrations on May Day. Saint Joseph is the patron saint of workers and craftsmen, among others.

Labor Celebration days existed in some European countries since the end of 18th century – sometimes on January 20 (France, 1793), sometimes on June 5 (France, 1867).

On 21 April 1856, Australian stonemasons in Victoria undertook a mass stoppage as part of the eight-hour workday movement. It became a yearly commemoration, inspiring American workers to have their first stoppage. 1 May was chosen to be International Workers' Day to commemorate the 1886 Haymarket affair in Chicago. In that year beginning on 1 May, there was a general strike for the eight-hour workday. On 4 May, the police acted to disperse a public assembly in support of the strike when an unidentified person threw a bomb. The police responded by firing on the workers. The event led to the deaths of seven police officers and at least four civilians; sixty police officers were injured, as were one hundred and fifteen civilians. Hundreds of labour leaders and sympathizers were later rounded-up and four were executed by hanging, after a trial that was seen as a miscarriage of justice. The following day on 5 May, in Milwaukee, Wisconsin, the state militia fired on a crowd of strikers killing seven, including a schoolboy and a man feeding chickens in his yard.

In 1889, the first meeting of the Second International was held in Paris, following a proposal by Raymond Lavigne [fr] that called for international demonstrations on the 1890 anniversary of the Chicago protests. On 1 May 1890, demonstrations took place in the United States and most countries in Europe. Demonstrations were also held in Chile and Peru. International Workers' Day was formally recognized as an annual event at the International's second congress in 1891. Subsequently, the May Day riots of 1894 occurred. The International Socialist Congress, Amsterdam 1904 called on "all Social Democratic Party organisations and trade unions of all countries to demonstrate energetically on the First of May 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 organisations of all countries to stop work on 1 May, wherever it is possible without injury to the workers."

In the United States and Canada, a September holiday, called Labor or Labour Day, was first proposed in the 1880s. In 1882, Matthew Maguire, a machinist, first proposed a Labor Day holiday on the first Monday of September while serving as secretary of the Central Labor Union (CLU) of New York. Others argue that it was first proposed by Peter J. McGuire of the American Federation of Labor in May 1882, after witnessing the annual labour festival held in Toronto, Canada. In 1887, Oregon was the first state of the United States to make it an official public holiday. By the time it became an official federal holiday in 1894, thirty US states officially celebrated Labor Day. Thus by 1887 in North America, Labour Day was an established, official holiday but in September, not on 1 May.

International Workers' Day has also been a focal point for demonstrations by various socialist, communist and anarchist groups since the Second International. It is one of the most important holidays in communist countries such as China, Vietnam, Cuba, Laos, North Korea, and the former Soviet Union countries. Celebrations in these countries typically feature elaborate workforce parades, including displays of military hardware and soldiers.