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The Roman calendar changed its form several times in the time between the foundation of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or 'pre-Julian' calendars.

The calendar1 used  after 46 BC is discussed under the Julian calendar. An inscription containing the Roman calendar, which predates the Julian reform of the calendar. Observe that it contains the months Quintilis and Sextilis, and allows for the insertion of an intercalary month.

The original Roman calendar is believed to have been a lunar calendar, which may have been based on one of the Greek lunar calendars. Roman traditions claimed that it was invented by Romulus, the founder of Rome about 753 BC. The earliest known version contained ten months, and started at the vernal equinox; however, the months by this time were no longer 'lunar':

· Martius (31 days)

· Aprilis (30 days)

· Maius (31 days)

· Junius (30 days)

· Quintilis (31 days)

· Sextilis (30 days)

· September (30 days)

· October (31 days)

· November (30 days) and

· December (30 days)

The calendar  year lasted 304 days and there were about 61 days of winter that did not fall within the calendar.

The Roman king Numa Pompilius (c. 715-673 BC, although his historicity is disputed) allegedly introduced February and January (in that order) between December and March, increasing the length of the year to 354 or 355 days. In 450 BC, February was moved to its current position between January and March.

· Martius (31 days)

· Aprilis (29 days)

· Maius (31 days)

· Junius (29 days)

· Quintilis (31 days)

· Sextilis (29 days)

· September (29 days)

· October (31 days)

· November (29 days)

· December (29 days)

· Februarius (28 days)

· Ianuarius (29 days)

But the length of the months from 45 BC onward were the same as the ones we know today.

Note that the later months are named based on their position in the original calendar — October comes from the prefix oct, meaning 'eight' (as in octagon). These names have been retained even though October is the 10th month of the modern calendar. Quintilis and Sextilis were later renamed July and August in honor of Julius Caesar and Augustus.

In order to keep the calendar year roughly aligned with the solar year, a leap month of 27 days, the Mensis Intercalaris, sometimes also known as Mercedonius or Mercedinus, was added from time to time at the end of February, which was shortened to 23 or 24 days. The resulting year was either 377 or 378 days long. The decision to insert the intercalary month, and its placement, was the responsibility of the pontifex maximus. On average, this happened roughly in alternate years.

1: 12 months or 355 days

2: 13 months or 377 days

3: 12 months or 355 days

4: 13 months or 378 days

5: 12 months or 355 days

6: 13 months or 377 days

7: 12 months or 355 days

8: 13 months or 378 days

The system of aligning the year through intercalary months broke down at least twice. The first time was during and after the Second Punic War. It led to the reform of the Lex Acilia in 191 BC. The details of this reform are unclear, but it appears to have successfully regulated intercalation for over a century. The second breakdown was in the middle of the first century BC. This breakdown may have been related to the increasingly chaotic and adversarial nature of Roman politics at the time. The position of pontifex maximus was not a full-time job; it was held by a member of the Roman elite, who would almost invariably be involved in the machinations of Roman politics. Because a Roman calendar year defined the term of office of elected Roman magistrates, a pontifex maximus would have reason to lengthen a year in which he or his allies were in power, or not to lengthen a year in which his political opponents held office. It was while Julius Caesar was pontifex maximus that the calendar was overhauled, with the result being the Julian calendar. The calendar reforms were completed during the reign of his successor, Augustus Caesar.

Months

The Romans had special names for three specific days in each month. The system appears to originally have been based on phases of the Moon (Luna), and these days were probably declared publicly when the lunar conditions were observed. After the reforms of Numa Pompilius, they occurred on fixed days. The named days were:

· Kalendae (Kalends) — first day of the month, from which the word "calendar" is derived; thought to have originally been the day of the new moon. Interest on debt was due on Kalends.

· Nonae (Nones) — depending on the month, could be the 5th or the 7th day; thought to have originally been the day of the half moon.

· Idūs (Ides) — depending on the month, could be the 13th or the 15th day; thought to have originally been the day of the full moon. The Romans considered this an auspicious day in their calendar. The word ides comes from Latin, meaning "half division" (of a month).

Months with Nones on the 5th and Ides on the 13th days:

January

February

April

June

August

September

November

December

Intercalaris.

Months with Nones on the 7th and Ides on the 15th days:

March

May

July

October.

These rules are summarised in the following mnemonic:

In March, July, October, May

The Ides fall on the fifteenth day

The Nones the seventh; all besides

Have two days less for Nones and Ides.

Other days in the month were normally unnamed. Days other than the Kalends, Nones and Ides were usually identified by counting down to the named days, in a way that is quite different from the modern Western calendar. The Romans did not count the days of the month retrospectively, looking back to the first of the month (that is: 1st, 2nd day since the start of the month, 3rd day since the start of the month). They counted forward to their named days. Also, to the distress of moderns trying to work out dates in Roman calendar documents, they counted inclusively, so that 2 September is considered 4 days before 5 September, rather than 3 days before.

The following example spells out how days were named for the pre-Julian September, which had only 29 days. It shows the Roman form of the date, the translation, and how we would say it today. The Romans used abbreviations: "a.d." = "ante diem" = "days before", "prid." = "pridie" = "the day before", "Kal" = "Kalends" etc.

Kal. Sept. = Kalends of September = 1 September

a.d. IV Non. Sept. = 4 days before the Nones of September = 2 September

a.d. III Non. Sept. = 3 days before the Nones of September = 3 September

prid. Non. Sept. = the day before the Nones of September = 4 September

Non. Sept. = Nones of September = 5 September

a.d. VIII Id. Sept. = 8 days before the Ides of September = 6 September

a.d. VII Id. Sept. = 7 days before the Ides of September = 7 September and so on till

a.d. III Id. Sept. = 3 days before the Ides of September = 11 September

prid. Id. Sept. = the day before the Ides of September = 12 September

Id. Sept. = Ides of September = 13 September

a.d. XVII Kal. Oct. = 17 days before the Kalends of October = 14 September

a.d. XVI Kal. Oct. = 16 days before the Kalends of October = 15 September and so on till

a.d. III Kal. Oct. = 3 days before the Kalends of October = 28 September

prid. Kal. Oct. = the day before the Kalends of October = 29 September

Kal. Oct. = Kalends of October = 1 October

Notice that by counting inclusively and by having a special name for the day before a named day the Roman calendar loses the possibility of saying: 2 days before a named day. Also, after the Ides, the date no longer mentions September, but is counting down towards October.

Some dates are sometimes known by the name of a festival that occurred on them, or shortly afterwards. Such dates are known for the Feralia, Quirinalia and the Terminalia (though not yet for the Lupercalia). These dates are all after the Ides of February, which suggests that they are connected with resolving an ambiguity that could arise in intercalary years: dates of the form a.d. [N] Kal. Mart. were dates in late February in regular years but were a month later in intercalary years. However, it is much debated whether there was a fixed rule for using festival-based dates. It has been suggested that dates like a.d. X Terminalia (known from an inscription in 94 B.C.) indicated that the year was intercalary, that it was not intercalary, or that it could be intercalary.

When Julius Caesar added a day to September, he added it to the end of the month, so as not to disturb the dates of religious festivals in September, but the effect was to increase the count of the day that immediately followed the Ides:

a.d. XVIII Kal. Oct. = 18 days before the Kalends of October = 14 September

As a result, the position of all the following dates in September got bumped up by one day. This has some unexpected effects. For example, the emperor Augustus was born on 23 September 63 BC. In the pre-Julian calendar this is 8 days before the Kalends of October (or, in Roman style, a.d. VIII Kal. Oct.), but in the Julian calendar it is 9 days (a.d. IX Kal. Oct.). Because of this ambiguity, his birthday was sometimes celebrated on both dates, i.e. (for us) on both 23 and 24 September.

Nundinal cycle

The Roman Republic, like the Etruscans, used a "market week" of eight days, marked as A to H in the calendar. A market was held on the eighth day. For the Romans, who counted inclusively, this was every ninth day, hence the market became called "nundinae", and the market cycle is known as the "nundinal cycle". Since the length of the year was not a multiple of 8 days, the letter for the market day (known as a "nundinal letter") changed every year. For example, if the letter for market days in some year was A and the year was 355 days long, then the letter for the next year would be F.

The nundinal cycle formed a basic rhythm of day-to-day Roman life; the market day was the day that country people would come to the city, and the day that city people would buy their groceries for the next 8 days. For this reason, a law was passed in 287 BC (the Lex Hortensia) that forbade the holding of meetings of the comitia (for example to hold elections) on market days, but permitted the holding of legal actions. In the late republic, a superstition arose that it was unlucky to start the year with a market day (i.e. for the market day to fall on 1 January, with a letter A), and the pontiffs, who regulated the calendar, took steps to avoid it.

Because the nundinal cycle was absolutely fixed at 8 days under the Republic, information about the dates of market days is one of the most important tools we have for working out the Julian equivalent of a Roman date in the pre-Julian calendar. In the early Empire, the Roman market day was occasionally changed. The details of this are not clear, but one likely explanation is that it would be moved by one day if it fell on the same day as the festival of Regifugium, an event that could occur every other Julian leap year. When this happened the market day would be moved to the next day, which was the bissextile (leap) day.

The nundinal cycle was eventually replaced by the modern seven-day week, which first came into use in Italy during the early imperial period, after the Julian calendar had come into effect. The system of nundinal letters was also adapted for the week. For a while, the week and the nundinal cycle coexisted, but by the time the week was officially adopted by Constantine  in AD 321 the nundinal cycle had fallen out of use. For further information on the week, see week and days of the week.

The year

In the early days, Romans denoted years by the names of the two Consuls who ruled each year and that system continued long after other ways of denoting the year were used. Later they began to count the years from the foundation of the City of Rome. There is no single agreed date for that but a Roman writer Marcus Terentius Varro fixed the date as what we would call 753BC and that is the standard I shall use here. Romans used the letters AUC after these dates (in Latin ab urbe condita - from the foundation of the city).

Our own starting point for the calendar is no more certain. We count years from the supposed date of the birth of Jesus of Nazareth. The letters AD after a date stand for Anno Domini - the year of our lord. This phrase was first used by a monk called Dionysius Exiguus in the year 531. But Dionysius was wrong about the date of the birth of Jesus - scholars now put that three years earlier in 4BC not 1BC.

By the time of Dionysius, the Roman Empire was ending - in the West anyway - so any Roman expression of the date using a year dating from the birth of Jesus of Nazareth is not correct Roman form. Oddly, the most common use of Roman numerals today is to do just that - to give the year AD.

There is a move to replace the letters AD for designating the starting point for our calendar. The phrase which they stand for, 'year of our lord', might offend people from other religions whose Lord, if they have one, was born in a different year. AD can now be written CE short for Common Era. And BC - which stands from Before Christ - can be written BCE.

The length of the year had been correctly determined by astronomers in different parts of the world many centuries before the Roman empire. But it was not until 46BC (708AUC) that the Roman emperor Julius Caesar introduced a calendar reform to recognise that the year lasted almost exactly 365.25 days.

NOTE:

1) Occasionally one reads the following story:

“Julius Caesar made all odd numbered months 31 days long, and all even numbered months 30 days long (with February having 29 days in non-leap years). In 44 BC Quintilis was renamed `Julius' (July) in honour of Julius Caesar, and in 8 BC Sextilis became `Augustus' in honour of emperor Augustus. When Augustus had a month named after him, he wanted his month to be a full 31 days long, so he removed a day from February and shifted the length of the other months so that August would have 31 days.”

This story, however, has no basis in actual fact.