According to the folks at www.curious.com:

Happy Labor Day! While most Americans consider Labor Day a homegrown holiday, it actually originated after an American machinist witnessed a national Labour Festival celebration in Toronto, Canada in 1882. It wasn't until 1894 that it became a U.S. holiday. On the heels of the deadly Pullman Strike where federal troops killed over thirty strikers, President Grover Cleveland quickly pushed a Labor Day bill through Congress to try and mend fences with the labor unions. The public didn't buy it, however, as Cleveland failed to even win his own party's nomination, losing the Democratic primary to William Jennings Bryan, largely on populist themes.