The Socialist celebration:
But the idea really gained ground in other parts of the world with the International Socialist congress of 1889 in Paris. It was the congress that designating it as an international labor day. While in the United States and Canada, Labor Day still continues to be observed on the first Monday in September, rest of the world observes it on May 1 or other dates.
http://www.theholidayspot.com/mayday/labor_day.htm