From Wikipedia:
International Workers’ Day, also known as Labour Day or Workers’ Day in some countries[1][2] and often referred to as May Day,[3][4] 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.[5][6]
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.[6] 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.”[7]
We used to celebrate May Day in elementary schools over sixty years ago!