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Answer: Change Residences
What we’re about to tell you is going to sound absolutely insane because, well, it was absolutely insane. In the city of New York, there was a long standing tradition dating all the way back to the time of the colonies (and maintained all the way into the 20th century) that everyone in the city who was changing residences in a given year would do so on the same day. If the mental picture you have of tens of thousands (if not hundreds of thousands) of New Yorkers all moving from one home to another on the exact same day is one of absolute chaos, you’d be right–and that was exactly how the people of the day described it.
So how did such a peculiar custom come to be and why did it fade into obscurity? Originally, the practice simply grew out of the free time afforded by May Day celebrations and festivals. People took advantage of the time to move from one home to another and get a fresh start with the spring. Eventually over time, a firmer system sprung up around the tradition with landlords giving notice of rent changes in February, giving tenants the rest of the quarter to find new housing if the rent changes were disagreeable to them. By 1820, the tradition was so firmly entrenched that the New York State legislature simply codified it into law, specifying that if a housing contract had no specific date of termination, then the contract was automatically terminated on the first day of May (unless the day fell on a Sunday, in which case the deadline was May 2nd).
Throughout the entire 19th century and well into the 20th century, changing customs (like the practice of many New Yorkers putting their belongings into storage and leaving the city during the heat of the summer) shifted the way housing contracts were written and rent was paid (like October 1st becoming a second Moving Day, for example). By the 1920s, the practice had decreased but didn’t ultimately die off until the outbreak of World War II. The sudden shortage of able-bodied men to move an untold number of trucks of belongings to and fro across the city (as so many were away at war) combined with the housing shortage that followed the war and the introduction of rent control all conspired to put an end to the tradition. By 1945, local papers declared Moving Day dead, marking the end of a hectic era.