The earliest known Mayday celebrations dates back to the pre-Christian times, in the Roman republic era, in the honor of Flora, the Roman goddess of flower. It was a traditional summer holiday in many pagan cultures across Europe. Later, with spread of Christianity, it lost its pagan culture, and turned into a secular celebration. Thus tradition of dancing around a pole called May pole, tradition of May Queen festival and crowning of Queen of May came into practice.
International worker's day was also chosen on this day by the communists and socialists to commemorate the Haymarket event in Chicago, but it is a different celebration from May day.
It may be noted, that some other days are also observed on the same day, like Loyalty day and Law day.