I was quite interested to find out the origins of Worker's Day.

Today is Worker’s Day, and for many of us it’s just a holiday. The date stems from the decision of a US federation of unions to set May 1 1886 as the date by which the eight-hour workday was to become standard. Organised labour had struggled for many years, in many different countries, to reduce working hours, after the Industrial Revolution commonly saw 6-day weeks of up to 16 hours. Women and children (but not men) were granted 10-hour work days in England in 1847, and the French achieved 12-hour work days in 1848.

The target had not been met as the deadline of May 1 1886 drew near, and rallies and strikes were organised. In Chicago, these culminated in the Haymarket Affair (also known as the Haymarket Riot, or Haymarket Massacre), in which a number of people were killed. It is this event that cemented the day in the modern calendar, as workers groups worldwide chose to demonstrate on May 1 in commemoration of the dead.
Full article on Ethical Co-op's blog