International Workers' Day is a celebration of labour and the working classes that is promoted by the international labour movement and occurs on May 1 every year. That day, May 1, is also the traditional European Spring holiday of May Day. Therefore, May 1 is a national public holiday in more than 80 countries, but in only some of those countries is the public holiday officially known as Labor Day or some similar variant. In the other countries, the public holiday marks the Spring festival of May Day.
Further, still other countries celebrate a Labour Day unrelated to International Workers' Day and on other dates significant to the labour movement in that country, such as the Labor Day in the United States which is on the first Monday of September.
May 1 was chosen as the date for International Workers' Day by the Socialists and Communists of the Second International to commemorate the Haymarket affair in Chicago that occurred on May 4, 1886.