Astronomy Answers
AstronomyAnswerBook: The Roman Calendar

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The Julian calendar as used by Julius Caesar differs from the currently used Western (Gregorian) calendar as follows:

1. in the language: Caesar wrote in Latin. Fortunately, the English names of the months are derived from the Latin names, so the Latin names can easily be recognized.
2. in the rules for leap years: the Julian calendar has a leap year every fourth year, without exceptions. Our current calendar has slightly fewer leap years.

3. in the numbers of the years: Romans used to say something like "in the second year of the first consulship of Julius Caesar", which is in the same spirit as "in the second year of the first term of office of Bill Clinton". If they used a continuous numbering scheme, then they often counted years A.U.C. (Ab Urbe Condita = since the foundation of the City [of Rome]). To translate a modern Western year count to a year A.U.C. you should add 753 to our year. For example, the year AD 1 is also year 754 A.U.C. The year −43, in which Caesar was assassinated, is also year 710 A.U.C.

   Also, numbers were written with letters, in the Roman fashion, and not with the digits that we use today.

4. in the numbers of the days of the month: the Romans did not count forward from the beginning of the month, like us, but counted backward from three different days of the month, the kalendae, the nonae, and the ides. They didn't talk of "the 3rd day of March", but of "the 5th day before the nonae of March". Also, they always included the starting and end points in the count, so what we would call "the day before" a Roman would call "the second day before". The date that we'd refer to as 15 December 2003 would be called "the 18th day before the kalendae of January of 2004" by a Roman.
5. in the location of the leap day. In our calendar, 29 February is the leap day, but in the days of Caesar the leap day was a second 23 February. A February in a leap year would contain 23 ordinary days, then the leap day, and then the remaining 5 ordinary days. I've been told that this manner of inserting the leap day is still used in the Roman Catholic calendar of saints.
6. in the name of some months: July is named after Julius Caesar. That month was called Quintilis before. August was named after Emperor August who came after Julius Caesar. That month was called Sextilis before. Caesar himself did not live to experience months called July or August, because he was killed a few months before the first July and a few years before the first August.

Below, I provide a Roman (Julian) calendar for the year 2003, which is not a leap year. In a leap year (for example, 2004), there is an a.d. bis VI Kalendas Martias between a.d. VI Kalendas Martias and a.d. V Kalendas Martias. Also, the below calendar already uses the month names July and August. To make it easier to read, I used both capital and small letters, though the Romans had only capital letters. The "a.d." at the beginning of each Roman date stand for "ante diem" which means "before the day". Romans always abbreviated that to a.d. They also often abbreviated the other words, such as Kal. for Kalendas, Non. for Nonas, Pr. for Pridie, and Mart. for Martias.

For convenience, I wrote our date (day number - month number) in front of each Roman date, but a Roman would of course not have done that.

I based this calendar on the description of the Roman calendar in the book "Onze Kalender" by W.E. van Wijk, published by de Wereld-Bibliotheek in 1955. For more information, see the Historical Calendars Page.

0.1. The Roman (Julian) Calendar for the year 2003

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Last updated: 2021-07-19