Why September Isn't the Seventh Month: The Origin of Month Names

A year that began in March, not January

Rome’s oldest calendar is credited to Romulus, the city’s legendary founder. It ran for ten months and 304 days, starting in March rather than January, a choice tied to the return of farm work in spring after the winter lull. The winter months, roughly from late December to the start of March, weren’t counted at all: a gap in the calendar the Romans simply left unnamed, since there was no field work in that stretch to mark off.

The ten original months were Martius, Aprilis, Maius, Iunius, then Quintilis, Sextilis, September, October, November and December: the last six, as their names suggest, were simply the fifth, sixth, seventh, eighth, ninth and tenth months of the year. Some sources note that the divine origins assigned to the first four months may partly be later reconstructions, not necessarily what the names meant when they were first chosen.

Numa Pompilius adds two winter months

It was Rome’s second king, Numa Pompilius, who added two months to cover the winter gap: Februarius and Ianuarius. With this addition the year reached 355 days, with an intercalary month inserted from time to time to keep it lined up with the seasons. February takes its name from the Latin “februa”, the purification rites held during that period, just before the arrival of spring.

The less intuitive detail is the order in which these two months were placed. February and January were initially added at the end of the calendar, after December; only later, according to one source, around 450 BC, were the two swapped, and January became the first month of the year. Not every source recounts this step with the same chronological detail, so the exact sequence deserves a degree of caution.

Example: picturing the earliest Roman calendar as a farming calendar helps explain why it started in March: Romans had no need to count days when the fields sat frozen and idle. Only once the calendar began serving religious and civic purposes as well as agricultural ones did it become necessary to name every single day of the year, winter included.

Why September isn’t the seventh month

Here’s the detail that trips people up most: September, October, November and December still carry their old numeral names, “seventh”, “eighth”, “ninth” and “tenth” month in Latin, even though today they sit in ninth, tenth, eleventh and twelfth place. Once January and February were moved to the front of the calendar, every month after them slid forward by two positions, and nobody bothered updating the names. That mismatch has survived intact into the calendar we still use, a small linguistic fossil sitting in plain sight.

Where the month names actually come from

Month	Origin of the name	Certainty
January	Janus, the Roman god of beginnings and doorways	Established
February	Latin “februa”, purification rites	Established
March	Mars, god of war, Romulus’s mythical father	Established
April	Two theories: the goddess Aphrodite/Venus, or the Latin “aperio” (“I open”)	Unresolved
May	Two theories: the nymph Maia, or the “maiores” (the elders)	Unresolved
June	Two theories: the goddess Juno, or the “iuniores” (the younger ones)	Unresolved
July	Renamed for Julius Caesar (originally Quintilis)	Established
August	Renamed for the emperor Augustus (originally Sextilis)	Established
September-December	Latin numerals (seventh-tenth month) from the old ten-month calendar	Established

For April, May and June, the historical sources won’t commit to a single explanation: each comes with two competing theories, with no indication of which one is correct. It’s a case where the honest answer is “nobody knows for certain”, rather than picking one version at random.

Julius Caesar and the “year of confusion”

Numa Pompilius’s calendar, built around lunar cycles with an intercalary month added at someone’s discretion, had a practical flaw: it drifted out of step with the actual seasons easily, especially when that extra month went unadded. Julius Caesar reformed the system, introducing a 365-day year with one extra day every four years, the leap year, added in February.

To reconcile the old, drifted calendar with the new system, 46 BC was artificially stretched to 445 days: historians call it the “year of confusion” for exactly this reason. The reform itself took effect on January 1, 45 BC.

July and August, the emperors’ months

Two months changed names to honor two politicians, not gods. Quintilis, the “fifth month” of the old calendar, was renamed Iulius in honor of Julius Caesar; one source places the renaming in 44 BC, though the exact year isn’t confirmed to the same degree by every source consulted. Sextilis, the “sixth month”, was renamed Augustus in 8 BC in honor of the emperor Augustus.

A detail undercuts a popular assumption: August wasn’t Augustus’s birth month, which was September. One source says the month was chosen instead because it was when Augustus first became consul and conquered Egypt.

The final touch-up: the Gregorian reform of 1582

The Julian calendar, a clear improvement on Numa Pompilius’s system, still overestimated the length of the solar year by about eleven minutes. A tiny error, but one that over centuries nudged the date of Easter noticeably, by roughly one day every 130 years. Pope Gregory XIII fixed the problem with the bull “Inter Gravissimas”, issued on February 24, 1582.

The most visible correction was a jump in the calendar itself: Thursday, October 4, 1582, was followed directly by Friday, October 15, 1582, dropping ten days from the calendar in one stroke. October was chosen because few major holidays or religious observances fell in that stretch. Alongside the jump came a new leap-year rule: the extra day is added only in years divisible by 4, with an exception for centuries not divisible by 400 (1600, for instance, stayed a leap year, but 1700 didn’t).

Example: Catholic countries adopted the new calendar almost immediately, between late 1582 and the following few years. England and Scotland, being Protestant, kept using the old Julian calendar until 1752: for nearly two centuries, a traveler moving between Rome and London had to keep track of a gap of several days between the two dates.

A linguistic fossil we still carry around

Behind twelve words we say without a second thought, the names of the months, sits more than two thousand years of layered reform: a ten-month farming calendar, the addition of two winter months, a solar reform from Julius Caesar, and a precision fix ordered by a pope. The numerals in September, October, November and December remain a visible trace of that first ten-month calendar, while July and August still carry the names of two men who wanted their mark written into time itself, quite literally.