Wikipedia will tell you that May Day was chosen as the date for International Workers’ Day by the Socialists and Communists. It was a Pagan holiday as well.
The mixing of the two has come to mean a day of Godlessness and revolution or rebellion yet recently, it’s become more of a symbol of Communism and Anarchy imbued with violent rebellion.
In these days of the rise of the violent leftist anti-Trump sentiment, I expect it to evolve into more of an anti-police, Chicago-style Haymarket Affair. Our larger cities, especially those with large, leftist college student populations, police forces had better prepare! The leftists have no hope without insurrection!