The curious case of December: Why the 12th month is the roman word for 10

The uncounted winter

In ancient times, the agrarian Roman society marked the new year in March, a month named after Mars, the God of War. March heralded both the planting season and the start of military campaigns. After December, the Romans entered a temporal limbo, leaving the barren winter months without nomenclature. The unassigned 61¼ days fell in the dead of winter, deemed unworthy of naming.
This oversight created challenges in aligning the calendar with the solar year, which spans approximately 365.25 days. Over time, the months drifted out of sync with the seasons, disrupting agricultural cycles and religious festivals.

The birth of January and February

To address this misalignment, Roman King Numa Pompilius in 713 BC reformed the calendar by introducing two new months – January and February. The months were appended to the end of the calendar, which still started in March. It was not until 153 BC that New Year’s Day was officially moved to January 1, a change made to enable newly elected consuls to take office sooner.

January (Jānuārius), was dedicated to Janus, the Roman god of doorways and transitions, often depicted with two faces looking toward the past and future, while February comes from the Latin word Februa, meaning “purification” or “to cleanse.” The name comes from the ancient Roman festival of purification, also called Februa, which was held on February 15 in the old lunar Roman calendar.

Numa, mindful of Roman superstition surrounding even numbers, attempted to eliminate them from the calendar. To achieve this, he reduced each of the 30-day months to 29 days. Numa’s reasoning was rooted in the mathematical principle that the sum of an even quantity of odd numbers yields an even total. With the lunar year averaging 355 days (or 354.367 days to be precise), Numa found himself with 57 days to allocate across the newly created months. He assigned January 29 days, consistent with the other months, while February received 28 days. This decision ensured that just one month out of the twelve contained an even number of days. February was designated as the “unlucky” month associated with rituals honoring the dead.

An extra month and political maneuvers

By the 1st century BC, the Roman calendar had descended into a state of utter confusion. Originally based on the cycles and phases of the Moon, it comprised 355 days—approximately 10¼ days shorter than the solar year. To realign the calendar with the seasons, an occasional intercalation of an extra month, known as Mercedonius, was introduced, adding either 27 or 28 days. This confusion was further exacerbated by political maneuvering. The Pontifex Maximus and the College of Pontiffs held the power to modify the calendar, often doing so to manipulate the terms of specific magistrates or public officials.

The longest 445-day year

By the 40s BCE, the Roman civic calendar had fallen three months out of sync with the solar calendar. Julius Caesar, advised by the Alexandrian astronomer Sosigenes, adopted the Egyptian solar calendar, which defined the solar year as 365¼ days. A new calendar was created, each having either 30 or 31 days, while February was the exception, containing 28 days in common years and 29 days in leap years. To align the civic and solar calendars, Caesar made a major adjustment in 46 BCE, extending that year to 445 days.

Unfortunately, Sosigenes’ calculations overestimated the solar year by 11 minutes and 14 seconds. By the mid-1500s, this error had compounded, shifting the seasonal dates by about 10 days from Caesar’s original design. In response, Pope Gregory XIII introduced a reform in 1582, realigning the calendar with the seasonal dates established in 325 CE, resulting in a 10-day adjustment. Since then, the Julian calendar has been replaced by the Gregorian calendar, with Great Britain making the transition in 1752.

Lingering names

As part of this Julian overhaul, Quintilis (the fifth month) was renamed July in his honor—a move cemented after his assassination. Augustus Caesar later followed suit, renaming Sextilis (the sixth month) as August. But what about the other months? Despite their numerical misalignment, September, October, November, and December remained untouched. Even as emperors came and went, these months stubbornly clung to their original names. This wasn’t for lack of trying. Some emperors saw the calendar as prime real estate for their legacies. Caligula tried to rename September to “Germanicus” in honor of his father. Nero fancied April as “Neroneus,” and Domitian wanted October to become “Domitianus.” None of these changes stuck.

The issue? Tradition is a hard habit to break. By the time these emperors made their proposals, the month names were so deeply embedded in daily life that changing them felt like more trouble than it was worth. Unlike Quintilis and Sextilis, which had clear political motives for renaming, the last four months lacked compelling reasons for a makeover.

In the end, the Romans let tradition prevail. The names of September, October, November, and December were part of the rhythm of life. Markets, festivals, and religious observances all revolved around these names. And so, thousands of years later, we still call December the twelfth month—even though it’s the tenth.