It was in 1887 that Oregon became the first state to establish Labor Day as a holiday, which it put on the first Saturday in June. Colorado, Massachusetts, New Jersey, and New York observed Labor Day on the first Monday in September that year. Then in 1889, the First (Paris) Congress of the Second Socialist International selected May First as a day for international celebration of the working man, no matter what day of the week it fell on. May first was chosen in commemoration of the Haymarket Massacre which occured in Chicago in 1886. In 1894, the first Monday in September was established as a federal holiday in the United States.
Why should the American working man celebrate Labor Day in September when the workers of the world are celebrating it on May first in commemoration of American Martyrs to the labor movement? This question is clarified by the fact that May first is observed unilaterally by workers (not by management), while the September holiday is enjoyed by all, perpetuating the myth that Labor and Management are both working together. The proclamation of Labor Day in September in the United States can only be interpreted as an effort to isolate the working American from his colleagues around the world, and obscure the history of what Management did to Labor in Chicago in 1886. Labor Day in the United States is better described as mocking than celebrating the working man in America.
[The immediate cause of the establishment of Labor Day as a holiday in September was to appease the working man after the Crushing of the Pullman strike in 1894.]
[A sanitized history of Labor Day can be found at History of Labor Day.]