It goes back to the day when in Chicago on May 4, 1886 workers demanded their employers to observe an eight-hour workday in a large protest. While the police tried to disperse the crowds, the demonstration then turned violent. The riot ended after mass shooting and a bomb was detonated in the crowd, and the event was labeled as the Haymarket Massacre.
Since then, the first of May was marked as a worldwide day for protests and riots when people demanded worker rights, and work environment reform.
To commemorate the day, some museums in Cape Town are closed.
In the U.S., Labor Day is celebrated on the first Monday in September. Click here to read more on the history on the holiday.