"Workers of the world unite!" - Karl Marx
The 1904 International Socialist Conference in Amsterdam, the Sixth Conference of the Second International, called on "all Social Democratic Party organizations 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." This established a tradition that has been carried on in some form or another ever since.
Being a traditional European spring celebration, May Day is a national public holiday in several European countries. The date is currently celebrated specifically as "Labor Day" or "International Workers' Day" in the majority of countries, including those that didn't traditionally celebrate May 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.