The Founding of the Roman Calendar

At a Glance

Central figures: Romulus, first king of Rome and creator of the original ten-month calendar; Numa Pompilius, his successor, who added the months of Januarius and Februarius and reformed the calendar into twelve months; Janus, the two-faced god of beginnings and doorways; and Julius Caesar, who in 46 BCE imposed the solar reform that became the Julian calendar.
Setting: Rome, from its founding in 753 BCE through the late Republic; the Palatine, the Regia in the Forum, and the temple of Janus on the Argiletum. Sources include Ovid’s Fasti, Plutarch’s Life of Numa, Livy’s Ab Urbe Condita, Macrobius’s Saturnalia, and Varro’s De Lingua Latina.
The turn: Numa Pompilius, seeking to bind Rome’s civic life to sacred order, added two new months to Romulus’s ten-month calendar and fixed the religious festivals to specific dates, transforming the calendar from an agricultural schedule into an instrument of state religion.
The outcome: Rome’s year grew from 304 days to 355, with Januarius placed under the protection of Janus and Februarius consecrated to purification rites; the pontifex maximus gained authority over intercalation, making the calendar a tool of political power.
The legacy: The Julian calendar reform of 46 BCE, which replaced the accumulated chaos of priestly intercalation with a fixed 365-day solar year, established the structure that survives - through one further adjustment by Pope Gregory XIII in 1582 - as the calendar the Western world still uses.

Romulus gave Rome ten months and told the city that was enough. The year began in Martius - the month of his father Mars - and ended in December, the tenth month, after which the days simply stopped being counted. Winter was a blank. The fields were dead, the legions stayed home, and the sixty-odd days between December and the next Martius belonged to no month at all. They accumulated like mud at the bottom of the Tiber, unclaimed.

This might have worked for a settlement of shepherds on the Palatine. It did not work for a city that intended to rule.

Romulus’s Ten Months

The calendar Romulus established carried the logic of a military and agricultural state compressed into a single count. Martius came first: war season, planting season, the month sacred to Mars. Aprilis followed - Varro connected its name to aperire, the opening of buds, though others claimed it for Venus through the Greek Aphro-. Then Maius for Maia, mother of Mercury, and Junius for Juno, or possibly for the iuniores, the younger men of fighting age. After these four named months, the Romans stopped bothering with gods and simply counted: Quintilis, Sextilis, September, October, November, December. Fifth through tenth.

Each month held either thirty or thirty-one days. The total came to 304. Romulus, according to Plutarch, considered this sufficient because it matched the cycle of agricultural labor. What fell outside that cycle fell outside the calendar. The days after December had no name. Shepherds tracked them by the weather; no magistrate tracked them at all.

The problem was practical before it was sacred. A city that signs treaties needs to date them. A city that wages war on a schedule needs to know when the campaigning season begins in relation to the actual position of the sun. Romulus’s calendar drifted, and the gap between the last day of December and the first of Martius grew shapeless, an administrative void that invited disorder.

Numa and the Two New Months

Numa Pompilius became king after the Senate’s long interregnum following Romulus’s death - or his disappearance, or his ascension as Quirinus, depending on who told it. Numa was Sabine, a man of rites rather than raids. He built the Regia in the Forum as the seat of the pontifex maximus. He closed the doors of the temple of Janus on the Argiletum to signal peace. And he reformed the calendar.

Numa added two months. Januarius he placed under the guardianship of Janus, the god who faces two directions at once - backward toward what has ended, forward toward what begins. The choice was deliberate. Janus presided over doorways, over the first moment of any undertaking, and Numa wanted the year itself to have a proper threshold. Februarius took its name from februa, the instruments of purification - strips of goat hide, salt cakes, woolen cloths - used in the rites of the Lupercalia and the Parentalia, the festival of the dead. February was the month of cleansing. The old year’s pollution had to be scoured away before the cycle turned.

The reformed calendar held 355 days, distributed across twelve months. Numa assigned twenty-nine days to Januarius and twenty-eight to Februarius, making February the shortest month and the only one with an even number of days - the Romans considered odd numbers more auspicious. He fixed the Calends as the first of each month, the Nones at the fifth or seventh, the Ides at the thirteenth or fifteenth. Every public festival received a permanent date. The Feriae Stativae - the fixed holidays - became anchored: the Lupercalia on the fifteenth of Februarius, the Equirria on the twenty-seventh of Februarius, the Liberalia on the seventeenth of Martius.

Where Januarius and Februarius fell in the sequence was disputed even among the Romans. Some sources placed them at the end of the year, after December, filling the old void. Others said Numa moved them to the beginning, pushing Martius to third place. The consensus in the late Republic and the time of Ovid was that Januarius had become the first month, but the old habit lingered. Consuls took office on the Ides of Martius until 153 BCE, when the date shifted to the Calends of Januarius - a change driven not by religion but by a military emergency in Hispania that required an earlier start to the campaigning year.

The Pontifex and the Drift

Three hundred fifty-five days is not a solar year. The gap between Numa’s calendar and the actual revolution of the earth around the sun was roughly ten and a quarter days per year. To compensate, the pontifex maximus had the authority to insert an intercalary month - called Mercedonius or Mensis Intercalaris - into the calendar, usually after the twenty-third of Februarius. When inserted, it ran for twenty-two or twenty-three days, and the remaining days of February were dropped.

In principle this should have kept the calendar roughly aligned with the seasons. In practice, the pontifices were politicians. They lengthened the year when their allies held office, so that those magistrates could serve longer terms. They shortened it when their enemies governed. By the late Republic, the calendar had drifted so far from the solar year that the harvest festival fell in what was astronomically still summer, and the month named for opening buds might coincide with snow.

Caesar’s 445-Day Year

Julius Caesar, as pontifex maximus and dictator, ended the chaos in 46 BCE. On the advice of the Alexandrian astronomer Sosigenes, he imposed a solar calendar of 365 days with a leap day every four years, inserted after the twenty-fourth of Februarius. To realign the calendar with the seasons, he declared that the year 46 BCE would last 445 days - the Romans called it annus confusionis, the year of confusion.

Quintilis was renamed Julius in Caesar’s honor after his assassination. Sextilis became Augustus under his successor. The other numbered months - September through December - kept their old names even though they no longer matched their numbers. The seventh month was now the ninth. The tenth was the twelfth. No one corrected this. The Romans, for all their legal precision, let the names stand, a fossil record of Romulus’s ten-month world embedded permanently in the calendar that outlasted the Republic, the Empire, and Rome itself.