May Day has three origins depending on the meaning.
International Workers' Day
In May 1886 in Chicago workers were holding a general strike to demand eight-hour work days. A peaceful march was being held which Chicago police were trying to break up. A bomb was thrown at the police who then rioted and opened fire on the demonstrators. It became known as the Haymarket Riot. Four years later the Second Socialists International designated May 1st to commemorate the Haymarket event as a key moment in the fight for worker's rights.
In 1894, the United States deliberately picked another day for its official Labor Day. The September Labor Day commemorates the date that Federal troops crushed the Pullman workers strike, killing 30. It is a date much more in tune with the American attitude towards worker's rights.
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