Why Is December The 12th Month Of The Year When Its Name Means 10?

A decade consists of 10 years, a decagon has 10 sides, and a decathlon comprises 10 sporting disciplines - so surely December should be the 10th month of the year. That it sits in 12th position is a quirk of Roman timekeeping and the combined fault of various emperors and popes.

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The exact origins of the Roman calendar are debated, although the most widely accepted account is that Romulus, the first Roman king, came up with the annual planner in the eighth century BCE. Based on the phases of the Moon, Romulus’s almanac divided the year into just 10 months, beginning in March and ending in December.

The seventh, eighth, ninth, and 10th months were therefore named in accordance with Latin numerical customs and given the prefixes sept, oct, nov, and dec. In that sense, the origins of December’s name make perfect sense.

What’s weird, though, is that Romulus left a 50-day winter gap in his calendar, with the period between December and March covered by a monthless abyss. According to most historians, it was his successor Numa Pompilius who corrected this by inserting the months of January and February into this wintry void, thus increasing the total number of months to 12.

However, rather than adapting the names of the existing months to reflect their new positions in the calendar, the Romans decided to leave September, October, November, and December unchanged, despite the fact that they were now all two months out of sync.

Centuries later, Julius Caesar overhauled this entire system and introduced the Julian Calendar, which was based on the Earth’s orbit around the Sun rather than the phases of the Moon. Even he, however, didn’t bother to correct the illogical sequence of names that designated the months of the year.

Fast forward to 1582 CE, and the Julian calendar was replaced by the Gregorian calendar, which remains the standard in most parts of the world to this day. Named after Pope Gregory XIII, the system is more accurate than its predecessor, which had included too many leap years and therefore failed to keep key religious holidays like Easter in their correct place.

For instance, rather than create an extra day every four years, the Gregorian calendar skips leap day whenever a year is divisible by 100 but not by 400, which means the year 2100 will have 365 days instead of 366. The introduction of this calendar also resulted in alterations to the lengths of some months and caused 10 days in October 1582 to completely disappear.

And yet, despite these impressive feats of calendrical acrobatics, even old Gregory seems to have been incapable of correcting the most glaring of nomenclative errors, leaving the months of September, October, November, and December all hideously out of whack with their positions in the yearly cycle.