Since it is Labor Day in a good part of the world, here is a video about the incidents at Haymarket Square following the call for workers to strike on May 1st. It picks up on May 3rd. The events would ripple throughout the world with labor movements looking at May 1st as a day of solidarity. When it came time in the US to pick a day, to help ease worker tensions after the death of several workers at the hands of the US Military and US Marshals, the date was moved to September as Grover Cleveland feared it would stir up negative emotions to the events at Haymarket Square.
Part 2 (4:50): http://youtu.be/8w-z8ud_9QU
Part 3 (5:17): http://youtu.be/VKkEl9XzjFc

From YT: Part 1. Public Broadcasting Service (PBS) looks at one of the incidents that gave rise to International Workers Day--May 1 or Mayday.

The video only deals with the Haymarket Square confrontation with police and the aftermath where eight labor leaders were charged with murder. Four were hanged, one committed suicide and three had their sentences commuted.

Labor organizers had called a national strike for an eight-hour work day on May 1, 1886. In Chicago, workers held a parade and rally with over 80,000 participants.

On May 3, 1886, striking employees of the McCormick Reaper Works clashed with replacement workers. Police retaliated against the striking employees, killing two.

On May 4th, 1886, a rally of anarchists and labor activists in Chicago's Haymarket Square in support of the McCormick strikers turned deadly. An unknown assailant tossed a bomb into a throng of riot police who were advancing on the rally, killing one instantly. In the chaos that erupted, seven policemen were killed, sixty injured, and civilian casualties were likely as high.

The eight men were arrested and charged with murder at Haymarket. Though they all opposed Chicago's elite businessmen, whom they believed stood for "starvation of the masses, privileges and luxury for the few," the eight held very different ideas about what action to take. Some advocated change through violence, while others believed progress could come via social engineering. Despite their different beliefs, the trial, convictions and sentencing that followed would unite these "Haymarket Eight" in history.

At a convention of the American Federation of Labor (AFL) in 1888 the union decided to campaign for the eight-hour day once again. May 1, 1890 was agreed upon as the date on which workers would strike for an eight-hour work day and to commemorate the earlier fight for an eight hour day.

In 1889 AFL president Samuel Gompers wrote to the first congress of the Second International, which was meeting in Paris. He informed the world's socialists of the AFL's plans and proposed an international fight for a universal eight-hour work day.
In response to Gompers's letter the Second International adopted a resolution calling for "a great international demonstration" on a single date so workers everywhere could demand the eight-hour work day.

In light of the Americans' plan, the International adopted May 1, 1890 as the date for this demonstration. It has been celebrated around the world as Mayday--International Workers Day-- ever since.