Labor Day has been the first Monday in September in the United States since President Cleveland, who had badly mishandled the Pullman Strike, signed a hastily passed bill in the summer of 1894 designating Labor Day as a federal holiday. Twenty-three states had already made the day a holiday.
As Slate magazine noted: "In 1894, after President Grover Cleveland ordered the brutal suppression of the Pullman Strike, he realized that he had to do something to curry favor with the labor movement, which viewed him with contempt. Worried that a May 1 holiday would encourage rabble-rousing in commemoration of the Haymarket Riot, he followed the lead of several states and made the first Monday in September a federal holiday in honor of the workingman. The political maneuver didn't achieve its desired effect, however: Cleveland lost the Democratic Party's 1896 presidential nomination to William Jennings Bryan.