The Haymarket Riots (or Haymarket affair or Haymarket massacre) was a demonstration on Tuesday, May 4, 1886, at the Haymarket Square in Chicago. It started out as a rally in support of striking workers. Someone threw a bomb at police as they dispersed the public meeting and that resulted in gunfire from the police, the deaths of eight police officers (most from friendly fire) and some civilians. The legal proceedings that followed got international press and eight “anarchists” were tried for murder. Four men were convicted and executed, and one committed suicide in prison, although the prosecution conceded none of the defendants had thrown the bomb.
U.S. President Grover Cleveland supported moving the holiday to a September date to avoid associations with the Haymarket riot and Socialist May Day associations. He signed a bill into law making the September Labor Day observance a federal holiday in 1894.
Unfortunately, Labor Day weekend in the United States has pretty much become another one of the holidays that has lost a lot of its original meaning. It is viewed as a day off and a long weekend and the unofficial end of summer.