The Roman Calendar

“Our knowledge of the Roman calendar comes from two different sources: from the living tradition and from ancient writers and documents. We still follow the Caesarian calendar, and the system of Roman dating (Nones and Kalends) was used until the sixteenth century (Ginzel III, 115).”
– E. J. Bickerman (1968)[3, p.101]

The two basic ancient sources that Bickerman listed were “Macrobius, Sat. (I, 13) and Censorinus (De die natali, written in AD 238)”. The fundamental modern source mentioned was Mommsen’s Romische Chronologie (1859).[3, p.101]

The kalends, ides, and nones are words used to refer to different parts of the Roman month. The first day of the month was known as the kalends. The day that was 16 days away from the next kalends was known as the ides. The day that was 9 days away from the next kalends was known as the nones.[1, p.25]

The fasti, nefasti, and feria are words used to refer to different types of Roman days. Public business was allowed on fasti but not on nefasti. Public holidays were known as feria.[1, p.25]

The founding of Rome is generally typically placed in the year 753 BCE and that year is the point from which all other years are counted on the Roman calendar.[2, p.13] There are a number of variation which I’m aware of for the year of Rome’s foundations and they range from 759-729 BCE.[3, p.77]

The Roman calendar originally had 10 months. The year began on the spring equinox with the month of March (Martius). The months were ordered:
1 – March (Martius)
2 – April (Aprilis)
3 – May (Mauis)
4 – June (Iunius)
5 – Quintilis
6 – Sextilis
7 – September
8 – October
9 – November
10 – December
Some time later two more months were inserted into the beginning of the calendar:
1 – Ianuarius
2 – Februarius
These two are the reason that the months September through December no longer correlate with their original positions on the calendar.[1, p.23]

All of the months in the Roman calendar had an odd number of days aside from February. This is due to their belief that even numbers were bad luck.[1, p.24] It does strike me as somewhat odd that they would have an even number of days in the month they dedicated to purification. If the month was about getting pure and discarding the bad, I don’t know why they would risk making it 28 days long. I don’t know if anyone has an explanation for this but if you do know someone who can explain this please let me know. I’ve made a note here just to remind myself that I found this peculiar and also to inform you that I’m interesting in hearing more. What is the basis for the idea that they thought even numbers were bad luck?

During Julius Caesar’s life, but before he reformed the Roman calendar into the Julian calendar, the Roman calendar had 12 months containing a total of 355 days. February was the shortest month and had 28 days. In 1968, Bickerman categorized the months by number of days. Here, I provide the information he relayed but I order the months according to Mondschein’s list above. Additionally, Bickerman provided more of the Roman month names than Mondschein and so I include those as well.[3, p .43]

1 – March (Martius) (31)
2 – April (Aprilis) (29)
3 – May (Mauis) (31)
4 – June (Iunius) (29)
5 – July (Quintilis) (31)
6 – August (Sextilis) (29)
7 – September (29)
8 – October (31)
9 – November (29)
10 – December (29)
11 – Ianuarius (31)
12 – Februarius (28)

“The problem with the Roman calendar was that it only had 355 days, which was solved by putting an intercalary month of 22 are 23 days after the first 23 days of February.”
Ken Mondschein (2020)[1, p.27]

The Roman calendar had a 4 year cycle that incorporated an intercalary month every other year. This quadrennial cycle contained a total of 1,465 days (355 + 378 + 355 + 377).[3, p.44] A solar year is about 365.24219 days and so 4 solar years would equal about 1,461 days. This means that every 4 years the Roman calendar would be off by 4 days, every 40 years by 40 days, and so on and so forth. As a side note pertaining to the 4 year sequence in the Roman calendar, it strikes me as odd that there would be a year with even numbers if the Romans indeed wanted to avoid even numbers.

At one point the Roman calendar was 67 days off from the solar year.[1, p.28]

There is only one surviving pre-Julian calendar known in existence. It is known as the “Fasti Antiates Maiores“.[4, p.102]


715-673 BCE: These years are when Numa, Rome’s legendary second king, is reported to have reigned. Ancient sources attribute the introduction of the Roman calendar to Numa.[3, p.45]

c.509 BCE: The Romans began driving a nail into one of the Temple of Minerva’s walls on the 13th of September every year to mark the year. It was around 509 BCE that their calendar placed 354 days in a year with an extra 22 days added every 2 years to make 1,460 days every 4 years.[2, p.14]

“The Roman Pontiffs, in exchange for private bribes, began to corrupt and falsify the Calendar Rolls.”
James Cecil Macdonald (1897)[2, p.14]

c.450 BCE: According to modern authors, this is when the Decemviri introduced the Roman calendar and also issued an intercalation law to try and correct the Roman calendar. Inevitably their law failed to correct the calendar.[3, p.45]

218-45 BCE: These years span from the start of the Second Punic War to the Julian reform. It was during this time period that, as Bickerman puts it, “the pontifices adjusted the calendar at will”.[3, p.45]

c.191 BCE: M’. Acilius Glabrio issued an intercalation law to try and correct the lag of the Roman calendar (to no avail).[3, p.45]

Late Republic: ab urbe condita (AUC) was introduced.[1, p.26]

46 BCE: The Roman calendar was behind the seasons by 90 days.[3, p.46]

C1st BCE: Julius Caesar revised the Roman calendar twice. The first time, two intercalary months were added between November and December. He then decided to abolish intercalary months and instead added 10 days to the 355 day year. He added 2 days to January, August, and December and 1 day to April, June, September, and November. He also added an intercalary day every 4 years (aka as a leap year).[1, p.28]

“The old calendar’s influence is seen in one interesting way: the Julian leap year was actually done by doubling February 24, and this is still maintained in the liturgical calendar of the Roman Catholic Church.”
Ken Mondschein (2020)[1, p.29]

Julius Caesar’s reformed calendar became known as the Julian calendar and this is the calendars that the Christian used when Christianity first started.

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[1] – Mondschein, Ken, and Neal Stephenson. On Time: a History of Western Timekeeping. Johns Hopkins University Press, 2020.

[2] – James Cecil Macdonald. “Chronologies and Calendars” (London, 1897).

[3] – E. J. Bickerman. “Chronology of the Ancient World” (1968). Accessed 2 Feb. 2021.

[4] – Hannah, Robert. Greek and Roman Calendars: Constructions of Time in the Classical World. Illustrated, Bloomsbury Publishing Plc, 2005. Accessed 10 Feb. 2021.

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