May Day is a public holiday usually celebrated on 1 May or the first Monday of May. It is an ancient festival of spring and a current traditional spring holiday in many European cultures. Dances, singing, and cake are usually part of the festivities.
In 1889, May Day 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. International Workers’ Day is also called “May Day”, but it is a different celebration from the traditional May Day. – Wikipedia