International Workers' Day, also known as Labour Day or Workers' 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 which occurs every year on May Day (1 May), an ancient European spring festival.

The date was chosen by a pan-national organization of socialist and communist political parties to commemorate the Haymarket affair, which occurred in Chicago on 4 May 1886. 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 8-hour day, for the class demands of the proletariat, and for universal peace."

The first of May is a national public holiday in many countries worldwide, in most cases as "Labour Day", "International Workers' Day" or some similar name – although some countries celebrate a Labour Day on other dates significant to them, such as the United States, which celebrates Labor Day on the first Monday of September.