In a Word: Why One-Third of Our Month Names Don’t Make Sense

Why do we call the 12th month a name that means ‘10th month’?


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Managing editor and logophile Andy Hollandbeck reveals the sometimes surprising roots of common English words and phrases. Remember: Etymology tells us where a word comes from, but not what it means today.

I begin every new article for this column with the same two sentences — you can see them italicized right above the paragraph you’re reading at this moment. While the first sentence just gives a quick explanation of what this column is about, the second one is, to me at least, more important: Etymology tells us where a word comes from, but not what it means today. I include this sentence for a very specific reason: to affirm that my column is not written to support the etymological fallacy.

The etymological fallacy is the belief that a word’s present meaning is (or should be) based exclusively on the word’s etymology. It isn’t; language changes. One of the most common examples of the etymological fallacy is the argument that decimate must only mean “reduced by one-tenth” because A) it’s part of the history of the word, and B) deci- (from the Latin decem) literally means “ten.”

I hear this argument all too often — I even made it in my younger, more foolish years.

My go-to argument for shooting down the decimate statement is this: If a word with decem at its root must retain that ten-ness in modern English, what does that mean for December?

That’s right: Etymologically speaking, December, the 12th month of the year, literally means “10th month.” If all words were defined by their etymologies, then October ought rightly be called December, and December should be Duodecimber.

In fact, the last four month are all “misnamed”:

  • September, from septem “seven,” is the 9th month.
  • October, from octo “eight,” is the 10th month.
  • November, from novem “nine,” is the 11th month.
  • December, from decem “ten,” is the 12th month.

How did our month-naming scheme get so scrambled? It goes back more than two millennia.

According to legend, the first Roman calendar was created by the founder of Rome himself, Romulus, in the mid-8th century B.C. (It may have been largely cribbed from an earlier Greek calendar.) This was, however, a lunar calendar, aligned to the phases of the moon rather than, like our modern calendar, to the Earth’s relationship to the sun.

This early calendar consisted of 10 months of 30 or 31 days each. The remaining time, about 61 days, was just an unlabeled gap through winter, when government and military actions were generally put on hold. Most of the month names used on this calendar still ring today:

  • Martius (March): This was the first month of the calendar year. Named for Mars, the god of war, Martius was when the military campaigns put on hold over the winter were reactivated.
  • Aprilis (April): The story behind this name is a bit murky. It could derive from the Latin aperire, meaning “to open,” which is what buds and some flowers do during April. It could also come from Apru, an Etruscan name for the Greek Aphrodite, goddess of fertility — a subject of concern during spring.
  • Maius (May): This month is probably named for Maia, a daughter of Atlas and mother to Hermes who is associated with growth. She is also one of the Pleiades, whose appearance in the night sky during this month coincides with the beginning of summer.
  • Junius (June): This month was named for Juno, the patron goddess of Rome who was said to watch after its women.

At this point, Romulus must have gotten tired of trying to match gods to months, so he just numbered the remaining ones: Quintilis, Sextilis, September, October, November, and December. So at this point in time — again, more than two millennia ago — September, October, November, and December actually were the seventh, eighth, ninth, and tenth months.

According to (sometimes questionable) histories, the Roman ruler Numa Pompilius is credited with giving those unlabeled winter months names, around 700 B.C.:

  • Januarius (January): Janus, who is depicted having two faces pointing in opposite directions, was the Roman god of beginnings, ends, and doorways.
  • Februarius (February): Februa was a Roman feast of purification held annually on what became the ides of February.

At this point, though, January and February were tacked on to the end of the calendar year, which still began in March. Several centuries would pass before New Year’s Day would shift to what we know, and it was entirely for political reasons: 153 B.C. was the first year that began on January 1, and it was moved here so that newly elected consuls could take office sooner.

In spite of this calendrical shift, the remaining months retained their original names — like today, December was the 12th month of the calendar year. Apparently even the ancient Romans understood that their language shouldn’t be limited by the etymological fallacy.

Through all this, the Roman calendar was still based on lunar cycles, which run about 29.5 days, and it was constantly becoming misaligned with seasonal changes and religious festivals. Readjusting the calendar required frequent intercalation — the addition of extra days or even months — a process that was fraught with political implications and was misused for personal gain.

This changed during the first century B.C. Under the reign of Julius Caesar, month lengths were altered and a new solar (or tropical) calendar was established; the first year under this Julian calendar began on January 1, 45 B.C. While the new calendar fixed a lot of problems and established leap years, it wasn’t perfect. You can find out more about that in “Bissextus: A Short History of Leap Years.”

An interesting result of this realignment, though, is that the months that were intercalated to bring the calendar into alignment extended the preceding year — which came to be known as ultimus annus confusionis “the last year of confusion” — to 455 days.

The months Quintilis and Sextilis still appeared on the original Julian calendar — as egomaniacal as Julius Caesar could be, he didn’t name one of the months after himself. Quintilis was renamed Julius (July) in 44 B.C. after the (recently) assassinated and deified emperor, whose birthday fell in that month. Similarly, Sextilis was renamed Augustus (August) in 8 B.C. in honor of Caesar Augustus, about 6 years after Julius’s successor’s death.

So as we begin September, the seventh ninth month, let it be a reminder that while a deep dive into etymology can be fun, interesting, and informative, its purpose is to help understand the history of language and language change, not to limit the present or future of it.


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  1. It’s great reading the origins of all 12 months of the year. You’re so right about the final third of the year with its mismatched numbering and naming. I find the whole thing fascinating amid the centuries old confusion, maybe partly because of it. You make it very clear with the italicized opening each week what this column is about, and stay true to it.


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