From colonial times up until World War II, May 1 was Moving Day, the one day a year when people in New York City moved. It's said the tradition came from the Dutch, who set out for Manhattan on May 1 and therefore celebrated each year by swapping homes on this day. Later, landlords had to notify tenants of rent increases on February 1, which would take effect three months later at 9 a.m. Tenants waited until May 1 to move, and the streets would be filled with "moving vans," Long Island farmers' wagons led by horses, clogging up the city streets and creating complete pandemonium.
More on this curious history here