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Roman calendar

This post is also available in: Polish (polski)

Stone plate with a Roman calendar depicting fasti – official list and calendar of Roman events. The original Roman calendar was lunar and was used in the Roman empire to reform the Julius Caesar, i.e. until 46 BCE inclusive.
Na licencji Creative Commons Uznanie autorstwa - Na tych samych warunkach 3.0.

In the first years of Rome, existed Romulus year, which according to ancient sources was divided into ten months1. The first month was March, which was named in honor of Romulus’ father – Mars. This year had only 304 days. After 30 days they received April, June, August, September, November and December. The remaining four months had 31 days. There were some discrepancies in the length of days of individual months in different parts of Italy. And so in Alba, March had 36 days, and September only 16, and October in Aricie as many as 39 days. It is now assumed that the Romulus year lasted much longer.

Another calendar reform falls on the reign of Numa Pompilius. Two new months were added then January (Januarius) and February (Februarius). The year was divided according to the course of the moon into twelve months, including leap years. Four months had 31 days, ie: March, May, July, October, February had 28 days, and the rest were 29 days each. The year in this calendar was 355 days long.
In the middle of the 5th century BCE, the decemvir commission wanted to introduce leap months and thus make the calendar more like the solar cycle. To this end, an additional month was introduced, which always began after February 23 or 24. He counted after 23 or 22 days and was called Mercedonius or Intercalaris. After the expiration of an additional month, the calculation of February continued.

From 191 BCE, priests had the right to add leap months at their own discretion under the law of Manius Acylius Glabrion. This resulted in complete arbitrariness in determining when to and when not to enter such a month. In the years 59–46 BCE, the pontiffs gave up their privilege, which was most often associated with the intervention of influential individuals. However, the changes that were introduced in previous years were irreversible. The calendar was such a mess that in 46 BCE the difference between dates and the correct date was 90 days. This in turn meant that the seasons did not coincide with the correct months. In order to organize the calendar, three additional months were introduced. In addition to the standard Mercedonius, two more months of 33 and 34 days were introduced between November and December. Macrobius called this year a year of confusion. Thus, the year 706 since the founding of Rome was 455 days.

Julian calendar

In the same year, Julius Caesar with the help of his astronomer Sozygenes from Alexandria introduced another reform of the Roman calendar. Sozyges calculated that each year consists of 365, 25 days. He determined the number of days for individual months, and made February a leap year every four years. The change, however, was not to add one day to February, as is done today, but to repeat it the same day. So every four years, every Roman lived the same day twice. It was set for February 24. The repeated day was called bissextilis and the leap year was also called bissextilis or bissextum. The beginning of the new calendar was set to January 1. January in the Julian calendar counted 29 days, February 28 days, 31 days each: March, May, July, September and November, and the remaining months were 30 days long. The previous duplicate month has been removed.

Shortly after Caesar’s assassination, priests counted a leap year not every four, but every three years. This resulted in further calendar days shifts. This was remedied by Octavian August, which in 8 BCE banned the introduction of a leap year for the next 12 years. This was related to the introduction, within 36 years of the new calendar, of 12 leap days, and not 9 as required by the reform. So it wasn’t until the 5th year CE that the reform of the calendar was successful. The earlier 50 years (from 45 BCE to 5 CE) are called “Julian error years” .

In the Roman calendar, the names of the first six months are derived from the names of the gods, with the exception of February, which took its name from the ceremony, while the remaining six are the order of the individual months from March, which is associated with the tradition of the Romulus calendar.



JanuaryIanuarius (29 days) – Janus month, god of all beginning


FebruaryFebruarius (28 days) – month of “purification” from februum – cleansing from blemish, carried out during holidays of Luperkalia


MarchMartius (31 days) – month of Mars, father of Romulus


AprilAprilis (30 days) – Venus month (Greek Aphrodite)


MayMaius (31 days) – month of Mai – Italian goddess, or Mai – Mercury’s mother


JuneIunius (30 days) – Juno’s month


JulyQuinctilis (31 days) – month five (quinque – five)


AugustSextilis (30 days) – sixth (sex – six)


SeptemberSeptember (31 days) – seventh (septem – seven)


OctoberOctober (30 days) – eighth (octo – eight)


NovemberNovember (31 days) – ninth (novem – nine)


DecemberDecember (30 days) – tenth (decem – ten)

During the empire and dictatorship of Caesar, the names of some months were changed in honor of the rulers. And so July from Quinctilis was changed to Julius (which was related to the deification of Julius Caesar), and August to August in honor of August Octavian. these are obviously changes that have survived to this day. Their successors also changed the names of months, but they were only ephemeris and in most cases they did not survive their reign. This was done, among others, by Domitian, who, after defeating the Germans and adopting the name Germanicus, changed the name of September to Germanicus, and October to Domitianus II. After his death, the original names of these two months were restored. The furthest he did, however, went Kommodus, which changed the names of all twelve months, replacing them with the names of its nicknames. So the months in his reign were: Amazonius, Invictus (invincible), Felix (happy), Pius (pious), Lucius, Aemilius, Aurelius, Commodus, Augustus, Hercules, Romanus, Exsuperantius (highlighted).

The Roman month was divided into three eight-day units, which were called nundins. Nundins did not fall out for specific periods, because the individual eight-day units were divided all year, not individual months. So in the calendar before 46 BCE the year had 44 nundins and 3 days off, while in the Julian calendar there were already 45 days and an additional 5 days off. The eight-day cycle was seven working days, while the eighth day was a day off from classes. Fairs were held then in big cities. During the empire, the fair was held only twice a month. Over time, the eight-day division was abandoned and replaced by a seven-day one. Emperor Constantine the Great introduced statutory Sunday as a non-working day. The names of individual days of the week are presented in the table below.

Days of the week


Mondaydies Lunae – Moon day


Tuesdaydies Martis – Mars Day


Wednesdaydies Mercurii – Mercury day


Thursdaydies Iovis – Jupiter’s day


Fridaydies Veneris – Venus Day


Saturdaydies Saturni – Saturn’s day


Sundaydies Solis – Sun day

Each day was divided into day and night. However, each night and day was divided into four more or less three-hour parts. Of course, the time units were different at different times of the year. In the case of the night, these four parts were called Christmas Eve (which in Latin means vigil).

In addition to the eight-day, and later seven-day division of the year, the Romans had an additional system of dating days each month. Each month had three specific dates that were the basis for dating the remaining days of the month. These dates were associated with subsequent phases of the moon. Their names were as follows:

  • KalendaeKalends, the first day of each month, they occurred during the first phase of the moon (s);
  • NonaeNones, fell out on the 5th or 7th day of the month, the day depended on the date of occurrence of the full moon;
  • IdusIdes occurred on the thirteenth or fifteenth day of the month.

The dates of non and id falling out were interrelated. Nony always fell out on the ninth day before idiots. So if the ides were 15, the nony had to be on the 7th of the month. The non and id fall out dates listed above were appropriate for the following months: March, May, July and October. In the remaining months Idy fell on the 13th and nony on the 5th of the given month. The day before ids or nonami was called pridie e.g. March 14=pridie Idus Martias. Other days were dated subtracting from the nearest date, including this day, e.g. March 13=ante diem tertium Idus Martias. The deadline for publishing ids and nonions was the responsibility of pontifex Minor, who watched the moon on calendars and on this basis announced the dates of ids and non.

In addition to dividing the year into months, and these in turn into the appropriate number of days, the Romans also introduced a new year dating system. They considered the foundation date of Rome to be the beginning. Earlier this year was set to 750 BCE In the 1st century BCE, the Roman historian Terentius Varro made new calculations and set the date of foundation of Rome for 753 BCE This way the date from the foundation of Rome began to be counted. Thus, the year 44 BCE can also be marked as 710 from the foundation of Rome, or shorter 710 AUC (ab urbe condita, meaning “from the founding of the City”). Until Varro, the Romans used a different system to mark the year. All dates were not related to a specific date but happened at the time of the consulate of a specific person. For example, the previously mentioned year 44 BCE was designated as the consulate of Gaius Julius Caesar (V) and Mark Antony. The Roman number in brackets next to Caesar means which of them was the consulate. In this case, the number 5 means that it was the fifth Caesar consulate.

  1. Macrobius mentions this in "Saturnalia" (I, 12, 39). Rome during the reign of Romulus (753-717 BCE) was to be based on a 10-month calendar; when two new months were introduced by the legendary king Numa (between March and December): January and February. In the book "Vademecum historyka starożytnej Grecji i Rzymu", in turn, we read that the original Roman calendar was based on the lunar cycle, where each cycle was roughly 28 days - which excludes the theory of ten months in the early royal period. During the Republic, the year was certainly 12 months old.
  • Jaczynowska Maria, Religie świata rzymskiego, Warszawa 1987
  • Szymański Józef, Nauki pomocnicze historii, Warszawa 2002
  • William Matthew O'Neil, Time and the Calendars
  • Leonardo Magin, Stars, Myths and Rituals in Etruscan Rome
  • Ewa Wipszycka, Vademecum historyka starożytnej Grecji i Rzymu, "PWN", 1983

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