Labour Day (Labor Day in the United States) is celebrated as a result of the hard fought battle to have the right not to work for 12+ hours in a day.
In the early 1880s Governments at the time had no guidance or legislative framework on working conditions. As a result labourers worked at the complete discretion of their employers. Many employees just worked until their employers said they could stop or they just dropped, whichever came first.
Most 19th century jobs were labour intensive work (not sitting at a computer writing this as I am) the 12 hours of physical labour daily greatly affected employee's health, and some felt it had to change.
On 18 August 1855, stonemasons in Sydney presented their employers with an ultimatum: they had six months to implement an eight hour work day—meaning eight hours for work, eight for recreation, and eight for sleep—or work would stop. The masons had circumstance on their side: thanks to the gold rush, the population was increasing rapidly and much of the country was in a building boom. As a result, Sydney had a shortage of skilled workers. By March 1856, they'd won an eight-hour work day, but there was a cost: the reduction in work time came with a reduction in pay.
One month later, stonemasons at the University of Melbourne followed suit. On 21 April they put down their tools and took off towards Parliament House. They were joined by other builders, and the crowd converged at the seat of the state government to demand their 60 hour work week be reduced to 48.
Surprising, the government agreed and the workers who were employed on government projects were the first organised employees in the world to achieve the coveted 8-hour day, with no loss of pay.
As each state achieved the 8-hour day at different times, Labour Day is celebrated on different dates across Australia.