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International Workers' Day, also known as Labor Day in some places, is a celebration of laborers and the working classes that is promoted by the international labor movement, anarchists, socialists, and communists and occurs every year on May Day, 1 May, an ancient European spring holiday.

The date was chosen for International Workers' Day by the Second International to commemorate the Haymarket affair, which occurred in Chicago on 4 May 1886. This Day has its origins in the labor union movement, specifically the eight-hour day movement, which advocated eight hours for work, eight hours for recreation, and eight hours for rest.

Being a traditional European spring celebration, May Day is a national public holiday in many countries, but in only some of those countries is it celebrated specifically as "Labor Day" or "International Workers' Day." Some countries celebrate a Labor Day on other dates significant to them, such as the United States which celebrates Labor Day on the first Monday of September.