The Coligny calendar is a bronze plaque with a written calendar, created in Roman Gaul during the 2nd century CE. It shows a 5-year cycle of a lunisolar calendar, with each year divided into 12 lunar months. An extra month is added every 2.5 years. This calendar is the most important piece of evidence used to understand how the ancient Celtic calendar worked.
The calendar was discovered in 1897 in France, near the town of Coligny in the Ain region (close to Lyon). It was found with broken pieces of a life-size bronze statue of a nude man holding a spear, likely meant to represent Mars, the Roman god of war. About 40% of the original calendar remains as fragments. It was engraved on a bronze tablet that originally measured 134.8 cm wide by 78.0 cm high. With the rim included, the tablet measured 52 by 32 unciae Drusianae (a unit of measurement). The text is written in the Gaulish language using the Latin alphabet, with Roman square capitals and Roman numerals. Based on the style of the writing and the statue, the plaque likely dates to the end of the 2nd century CE. However, copying errors suggest the calendar itself is older. It is now displayed at the Gallo-Roman Museum of Lyon-Fourvière.
Eight small pieces of a similar calendar were found at the double-shrine of Villards-d'Héria. This calendar does not have the holes of a peg calendar, like the Coligny calendar, but it has the same notations. It is now kept at the Musée d'Archéologie du Jura in Lons-le-Saunier.
List of months
The names of the twelve lunar months are believed to be Samonios, Dumannios, Rivros, Anagantios, Ogronios, Cutios, Giamonios, Simivisonnios, Equos, Elembivios, Edrinios, and Cantlos. These names appear in the genitive form, such as SAMONI, DVMANNI, RIVRI, in the calendar's internal notes. The first extra month added to the calendar may be called QVIMON, possibly short for Quimonios. The second extra month is thought to be named …antaran, starting with the letters B, R, or S.
Samonios means "summer" and comes from the word samo-. Scholars Xavier Delamarre and Ranko Matasović suggest that Samonios may have originated from the Proto-Celtic samoni-, meaning "assembly" or "feast of the first month of the year," possibly referring to a gathering of living and dead people. This name may conflict with Giamonios, which relates to the Gaulish word giamo-, meaning "winter."
Samonios and Giamonios split the calendar into two six-month seasons: summer and winter. Each season begins with a festival lasting several days, marked by the word IVOS. This shows an early version of the same seasonal divisions found in later Celtic traditions: "The year was once divided into two parts—summer starting on Beltaine (the first day of May), and winter from Samuin to Beltaine."
To adjust for differences between lunar and solar years and match the month names to the seasons of the solar year, Samonios may have started around the first quarter moon in May or June. If linked to modern Gaelic festivals like Beltane, Lughnasadh, Samhain, and Imbolc, Samonios might have begun around the first quarter moon in April or May.
The lunar month
The Coligny calendar, as reconstructed, had 16 columns and 4 rows. Two extra months each used half a column, creating a table that showed all 62 months in a 5-year cycle. Scholars debate whether these 5 years are part of a 19-year or 30-year Metonic cycle, but the full length of the calendar remains uncertain.
Each lunar year has 12 months. Six months have 30 days, five months have 29 days, and one month has 29 or 30 days, depending on the moon’s position. Since synodic months last about 29.53 days, the calendar may adjust by removing a day from the 30-day Equos month. The length of Equos might have been determined by observing the first quarter moon, which marked the start of months.
The first extra month appears in year 1, before the month Samonios. The second extra month occurs between Cutios and Giamonios in year 3. The second extra month has 30 days, but the first extra month’s length is debated—some believe it had 29 days, others 30. These extra months copied day numbers from other months across the 5-year cycle to create their own notations.
McKay suggests the first extra month had 29 days because copying the 30th day of a 29-day month, Cantlos, in year 1 would reference a day that does not exist (DIVERTOMV). Olmsted argues it may have had 30 days, as it is labeled as a MATV month. The broken part of the Roman numeral for the last day might have been slanted to look like XV instead of XIIII.
The start of the lunar month
The month is split into two parts, with ATENOVX marking the middle. The first part has 15 days, called "cóicthiges" in Old Irish, meaning "fifteen-days," and "coicís" in Irish, meaning "fortnight." The second part has either 14 or 15 days. If there are 14 days, a "virtual" day called DIVERTOMV is used instead of the 15th day for notations.
Pliny the Elder wrote that the Celtic month began on the "sixth day of the new moon."
Classical writers counted days starting from the day the moon was first seen. This means the sixth day would be when the moon was a quarter full. The calendar’s internal notes match Pliny’s description, focusing on the middle three days of each half-month. This means the full moon would appear on days 7, 8, or 9, and the new moon would appear on days 7a, 8a, or 9a.
Full reconstruction
A complete reconstruction of the calendar by McKay (2020) includes the most recent details about special marks added to the calendar and the triple marks. Olmsted (2001) created an earlier version of the calendar that matches written notes with pictures. RIG III (1986) provided an earlier detailed explanation of terms and a reconstruction of the calendar.
If based on the Metonic cycle, the calendar can be created using four 5-year cycles. The first year is skipped, and the months called Equos are 30 days long in the first and fifth years of each cycle. All days and notes on the calendar are based on both the moon and the sun, and they fall within a 36-day range of a solar date.
The Metonic cycle lasts 6,940 full days. Over time, this cycle causes the sun and moon to be slightly out of sync, by about 0.398396 days for the sun and 0.311620 days for the moon. Because the calendar tracks the moon’s phases, a 30-day Equos month would need to be shortened to 29 days roughly every 61 years. After 219 years, the calendar would be one day ahead of the sun, requiring a 30-day month to be skipped after 6,569 years or a 29-day month after 6,350 years.
The calendar can also work as a 30-year cycle, using six 5-year cycles. A 30-day special month is skipped once every 30 years. If part of a 30-year cycle, the calendar would be 0.1515 days ahead of the moon’s phase, requiring one day to be removed from a 30-day Equos month roughly every 198 years.
However, the months inside the calendar show a larger difference in accuracy for the moon’s phase, varying by nearly 48 hours (from 1.44 to −0.65). This makes tracking the moon’s phase over 30 years less accurate. The difference between the moon and sun is larger at 1.4172 days, requiring a 30-day month to be skipped every 198 years.
This quick shift compared to the solar year would increase the difference between the moon and sun, creating a total shift of 75 days before an adjustment. This could cause seasonal festivals to be moved up to two and a half months earlier.
Sample month
The month of SAMONIOS in year 2 is the only month with no missing parts, keeping all its writings intact. Most patterns of the writings are known and can be put back together with some effort, though why they were used is still unknown.
The title begins with the letter M, showing it is a month, followed by the name and its type. For example, M SAMON MAT means "M(onth) Samon(ios) Lucky/Good." Days are arranged in rows, with the word ATENOVX, meaning "Renewal," separating the month into two halves. Each day has four sections: one for a peg-hole, one for a Roman numeral, one for a triple-mark, one for the day type, and space for extra notes. The letter M in the day type appears in the triple-mark section.
Numbers for the days are written using Roman numerals with additive notation. After ATENOVX, the count starts over. The triple marks can have no value, or they can be ƚıı, ıƚı, or ııƚ. Some marks include the letter M after the triple mark or in its place, which relates to the next day's type. Day types are lined up vertically with D for "day" or N for "night." Additional notes on days explain if days were swapped, give more details about the day type, or mention festivals. The notation PRIN LOVDIN covers both the type and the extra notes.
Calendar notations
Several different symbols, each with their own patterns, are placed in order on the 12 lunar months of the calendar. These symbols interact with each other based on certain rules, sometimes replacing previous symbols. After the basic symbols are set, many symbols are moved to other days, creating a confusing visual pattern. Finally, the days of the intercalary months are filled with symbols copied from specific days in the 12 regular months.
Over the past 100 years, researchers have gradually identified the symbols, their patterns, and how they interact. What follows is a general, but not complete, description of each symbol.
Each month has two halves. The first half includes days numbered I to XV (1 to 15). The second half includes either I–XV (1–15) or I–XI (1–14), with the 15th day marked as DIVERTOMV. The term ATENOVX is placed between the two halves. The patterns of the symbols act as if the 30th day always exists. This means some months have only 29 days in practice, but all months are considered to have 30 days conceptually.
Six months are marked in their headers as MAT "good, auspicious," and six as ANM[AT] "not good," based on comparisons with Middle Welsh and Old Irish words.
The summer season has 4 MAT and 2 ANMAT months, while the winter season has 2 MAT and 4 ANMAT months. MAT months have 30 days, and ANMAT months have 29 days, except for Equos, which can have 30 days in years 1 and 5.
The order of MAT and ANMAT months is determined by the seasons. MAT months begin in summer with Samonios, and ANMAT months begin in winter with Giamonios. This order helps determine the triple mark and PRINI LOVD/LAG symbols across the days of the month.
Days in MAT months are initially labeled M D (or MD) "good/auspicious day," and days in ANMAT months are labeled D "neutral day." The terms M D and D refer to daylight hours, in contrast to N "night." Any symbol with N overwrites the full daytime symbols, including triple marks, M D, D, and D AMB.
D AMBRIX RI, usually shortened to D AMB, marks an inauspicious day. It appears only on Days 5 and 11 in the first half of the month, when the moon is more than half full, so these days are mostly free of inauspicious symbols. In the second half, D AMB appears on every odd-numbered day except Day 1, which is not considered odd or even. The use of odd numbers for inauspicious days is linked to ANMAT months, which have 29 days. This reflects a Celtic cultural view, as the Romans believed odd numbers were auspicious.
Triple marks are a series of ogham-like symbols. Each month starts with three symbols placed over three days (ƚıı, ıƚı, or ııƚ), followed by three days with no symbols. These marks only appear on days labeled D (daytime) and never on N (nighttime), suggesting they divide the daytime into three parts.
Triple marks are the most complex symbols, with three main patterns. They do not always repeat across years. The first pattern places triple marks starting on the same day as the first PRINI term in the month, moving one day later in each following MAT or ANM month. For example, the first triple marks in Year 1 begin on Days 1–2–3 of Samonios, Days 2–3–4 of Rivros, and so on, following the MAT sequence. The same pattern for ANM months begins on Days 1–2–3 of Giamonios in Year 3.
A second pattern determines which triple marks from the first pattern appear each year, meaning the same day/month may have different symbols in different years. A third pattern adds an IIT on Day 21 (6a), the last day of the visible moon. If another mark is already present, it combines to form TIT, ITT, or IIT.
As other symbols are added, triple marks change. Symbols with N overwrites the entire "day" notation, such as IIT MD becoming just N, or ITI D AMB becoming just N. Days are moved and exchanged, often overwritten or lost. Intercalary borrowed days are marked with N, creating visual confusion.
PRINI LOVD and PRINI LAG follow the same month patterns as MAT and ANM months. If they appear on a day with triple marks, they replace the triple marks and any M D, D, or D AMB symbols. The PRINI LOVD of SIM 5 is later overwritten by N INIS R. Exchanges can move PRINI LOVD into ANM months and vice versa.
The term N INIS R appears scattered across the lunar year. Its distribution is not fully understood. Most instances occur in the seven months of the Samonios season plus Giamonios. It avoids days marked with IVOS "festival." It appears on seven nights when the moon is absent (the dark moon of 7a–8a–9a) and avoids the full moon on Day 8 and the first visible moon on Day 10a. This suggests it may relate to star-based predictions.
The term IVOS "festival" appears in groups of 3 to 9 days, marking festival days. Most festivals span the end of one month and the start of the next. Four of these groups divide the year into four parts, similar to the four major Celtic festivals, but centered on Day 1 every three lunar months instead of every three solar months.
There are also three other IVOS festivals on the calendar.
The term SINDIV IVOS "this day a festival" appears only three times: DVM 2a, SIM 9, and AED 25. These days likely mark events of great importance.
TIOCOBRIXTIO appears on three days: SIM 7, AED 8, and CAN 15. It may indicate days of special significance. Olmsted suggests it could mean "A day in place of Day 30," possibly replacing the missing Day 30 of Cantlos.
At this point, most symbols are placed on the calendar. A major feature of the calendar is the movement of one day’s symbols to another day. This disrupts patterns, making the calendar look random. Swapping days according to different patterns involves 870 days over 5 years.
Some patterns involve two days swapping their symbols. As symbols move, they retain information about their original position. Most moves are to the same day of the month, so only the month name (in the genitive) is added. Swaps between different days require both the original day name and month to be added.
For the 12 lunar months after an intercal