Dan Lewis, Now I Know, September 26, 2013:
There are roughly eight million residents of New York City. According to a 2010 article in the New York Times, 70% of them rent. That's about 5.5 million people, each of whom could be moving to a new place once their current lease runs out. Yet despite the incredible number of both apartments and renters, in general, the process of renting an apartment in New York City (and particularly in Manhattan) occurs over a short period of time. Typically, a renter starts his or her search only a few weeks before moving, checks out a dozen or so places over a day or two, and upon finding something suitable, quickly readies an application and bank checks to cover fees. The whole system — to use the term loosely — is a frenetic ordeal which hardly seems like it could be worse.
But until about seventy years ago, it was. Because almost everyone moved at the same time.
Sometime in the 1600s or early 1700s, New York City developed an odd tradition. Leases, across the city, expired at 9:00 AM on the first day of May. The origins of this tradition are unclear. Wikipedia cites to two different sources, one of which references the English celebration of May Day (explained here), another which claims that the Dutch settlers originally came to Manhattan on May 1st, and the tradition is borne out of that. In any event, the cartoon above, from 1856, encapsulates the madness — thousands upon thousands of people taking to the streets, with all their stuff, moving from one apartment to another, all on the same day. Davy Crockett observed the phenomenon in 1834, as retold by Futility Closet:
By the time we returned down Broadway, it seemed to me that the city was flying before some awful calamity. 'Why,' said I, 'Colonel, what under heaven is the matter? Everyone appears to be pitching out their furniture, and packing it off.' He laughed, and said this was the general 'moving day.' Such a sight nobody ever saw unless it was in this same city. It seemed a kind of frolic, as if they were changing houses just for fun. Every street as crowded with carts, drays, and people. So the world goes. It would take a good deal to get me out of my log-house; but here, I understand, many persons 'move' every year.All in all, this "moving day" was a terrible idea.
Being an informal one, and a custom at that, it should have been an easy one to change. And it wasn't universal. That is, not everyone's lease ended on May 1, so it should have been pretty easy for landlords or renters to demand a different date if given that one by the other. But the tradition persisted, to the point that in 1912, Harper's Weekly imagined a world with automated, flying "moving stations," as seen here. Moving Day was entrenched in a city which to a person, with few exceptions, hated it.
It quite literally took a world war to end the practice. When the United States mobilized its citizens in World War II, it created a shortage of able-bodied men stateside, making it nearly impossible to find someone to help you move on "moving day." Tenants stayed on past their leases and the practice of relocating on that date began to erode. And then, in 1945, the end of the war dealt "moving day" a death blow. As the New York Times reported in October of that year, troops returning from abroad created a major housing shortage in the city, which "turned 'moving day' into a myth."
Bonus fact: "Everybody moves!" days aren't a lunacy unique to New York of lore. July 1 in Montreal has the same thing happening. The operative word there is "has," because (as of 2013) it's still a thing there.