December is the twelfth and final month of the 12-month Gregorian calendar. Yet its name comes from decem, the Latin word for ten. So why is the 12th month called “10?”

Originally, the Roman calendar featured 10 months, was a lunar calendar that stretched from March through December, consisting of 304 days. The remaining winter days after December were not counted as any month. Supposedly, according to the Roman historian Livy, during the reign of┬áNuma Pompilius (715-673 B.C.), two winter months, Januarius and Februarius, each with twenty-eight days, were added. Instead of renaming all of the other months, though, they simply were pushed forward. Thus, December, even though its name is from “ten,” became the twelfth month (and, obviously, September, November, and October all got pushed forward, too, without a name change). Censorinus states that fifty days were added to the calendar, with one day taken from each month of thirty days. The lunar year thus consisted of 354 days. However, due to superstitions regarding even numbers, a day was added to January, making the total for the year 355.

These legendary accounts are generally disregarded by historians. It is thought that the actual lunar calendar reform, with the inclusion of two new months, occurred when the Decemviri in 450 BC published a revised calendar as part of the Twelve Tables, Rome’s first code of law. There have been many reforms made to what has become the standard calendar today. But none of them have updated the names of the months, and, since nothing has changed in over 2,500 years, I think the misnomer is a permanent one.

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