Coligny Calendar

In 1897, the Gaulish Coligny Calendar was discovered in Coligny, France along with the head of a bronze statue of a youthful male figure. It is believed to date from around 50 BC, and appears to be the remains of a Romanized Gaulish model of a Celtic lunar and solar calendar (lunisolar) . It displays a cycle of approximately five years on 62 tables. Unlike our present-day calendar which dates back to Julius Caesar, this system used the accurate period of the moon’s orbit around the Earth (the lunar month) to measure the passage of time. Each lunar month corresponds to 29.53 days. In this  Gaulish model, the month was divided into two 15-day periods.

It was engraved on a bronze tablet, preserved in 73 fragments, that originally was 1.48 m wide and 0.9 m high or approximately 5 feet (1.5 m) wide by 3½ feet in height. Based on the style of lettering and the accompanying objects, it probably dates to the end of the 2nd century . It is written in Latin inscriptional capitals, and is in the Gaulish language (Duval & Pinault). The restored tablet contains sixteen vertical columns, with 62 months distributed over five years. 

The French archaeologist J. Monard speculated that it was recorded by druids wishing to preserve their tradition of timekeeping in a time when the Julian calendar was imposed throughout the Roman Empire. However, the general form of the calendar suggests the public peg calendars (or parapegmata) found throughout the Greek and Roman world. 

A solar year, the time taken by the Earth to circle the sun, or one revolution of the sun about the Vernal Equinox, is nominally 365 days. Twelve revolutions of the moon, however, equals only 354 days. It was therefore necessary with the Coligny calendar to make two adjustments: first, using alternate months consisting of 29 and 30 days; second, adding a month every 2 1/2 or 3 years to link up the shorter lunar year of 354 days to the solar year of 365 days.

In Celtic legend the new year started on the moonrise of the first last-quarter moon after the autumnal equinox. In the Celtic regions of Britain and Ireland, the tradition was held that the new year started at Samhain (November 1) so that it would always occur on the same day of the solar cycle. A different calendar system from the one found in Gaul, one that reconciled the lunar and solar yearly cycles, was then in use . The year consisted of 13 months, 12 of them roughly equivalent to our modern calendar, with the inclusion of a  short, three-day month at the end of October leading up to the new year. It is in this arrangement of months that Celtic cosmology and Druid philosophy are linked through the Ogham alphabet with its 13 calendar trees.

The Continental Celtic calendar as reconstructed from the calendars of Coligny and Villards d’Heria had the following properties:

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  it was a lunisolar calendar, attempting to synchronize the solar year and the lunar month.
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  the months were lunar. Scholars disagree as to whether the start of the month was the new moon or the full moon, or per Pliny and Tacitus perhaps even the First Quarter.
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  the common lunar year contained 354 or 355 days.
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  the calendar year began with Samonios, which is usually assumed to correspond to Old Irish Samhain, giving an autumn start to the year. However, as Samon is Gaulish for summer , this assumed start is disputed. Le Contel and Verdier (1997) argue for a summer solstice start of the year. Monard (1999) argues for an autumn equinox start. Bonsing (2007) argues for a May beginning consistent with Irish Beltaine, and Fennian literature, notably Joyce (2000). 
  the entry TRINVX[tion] SAMO[nii] SINDIV “three-nights of Samonios today”) on the 17th of Samonios suggests that a festival of Samhain was considered to last for three nights. 
  the solar year was approximated by the insertion of a 13th intercalary month every two and a half years (unlike the Islamic calendar, where the calendar year keeps shifting in relation to the solar year). The additional months were intercalated before Samonios in the first year, and between Cutios and Giamonios in the third year. The name of the first intercalary month is not known with certainty, the text being fragmentary; the second intercalary month is Ciallos bis Sonnocingos (Lambert p.116) 
  the months were divided into two halves, the beginning of the second half marked with the term Atenoux. The basic unit of the Celtic calendar was thus the fortnight or half-month, as is also suggested in traces in Celtic folklore. The first half was always 15 days, the second half either 14 or 15 days on alternate months (similar to Hindu calendars). 
  months of 30 days were marked Mat(os), lucky. Months of 29 days were marked Anm(atos), unlucky. 
  a simple five year cycle would be insufficiently accurate; the sequence of intercalary months is completed every thirty years, after five cycles of 62 lunations with two intercalary months each, and one cycle of 61 lunations, with a single intercalary month, or after a total of 11 intercalary months. This assumes that there are exactly 371 lunations in 30 years, which is accurate to a one day every 20 or 21 years on average (this is less accurate than the Julian calendar, which shifts a day in about 130 years, but which ignores lunar months). It may be assumed that the “30 years cycle” was not prescriptive, and that an extra month would have been omitted as the need arose (i.e. some 300 years after the calendar’s inception). 
  The interpretation of atenoux as “returning night” is improbable (Delamarre p.58) and “renewing” would seem more probable; thus the month would start at new moon and atenoux would indicate the renewal, ie the full moon.

[edit] Pliny the Elder
The Natural History of Pliny the Elder states, in a discussion of Druidic gathering of mistletoe (Pliny NH 16.95):

The mistletoe, however, is but rarely found upon the robur; and when found, is gathered with rites replete with religious awe. This is done more particularly on the sixth day of the moon, the day which is the beginning of their months and years, as also of their ages, which, with them, are but thirty years. This day they select because the moon, though not yet in the middle of her course, has already considerable power and influence; and they call her by a name which signifies, in their language, the all-healing.

This comment supports the grouping of five-year Coligny calendar periods into thirty-year ages, with the loss of one intercalary month per age to more accurately align the solar and lunar cycles.

[edit] Julius Caesar
Julius Caesar in The Gallic Wars states (Caesar, DBG 6.18) that days, months, and years start with a dark half followed by a light half.

All the Gauls assert that they are descended from the god Dis, and say that this tradition has been handed down by the Druids. For that reason they compute the divisions of every season, not by the number of days, but of nights; they keep birthdays and the beginnings of months and years in such an order that the day follows the night.

This is consistent with a month starting at the dark of the moon, or at the sixth day of the moon as with Pliny; it is inconsistent with a month starting at full moon, as mentioned in many Neopagan discussions of the Coligny calendar.

The festivals of Beltane (Giammonios full moon) and Lughnasadh (Elembivios full moon) have been claimed to be indicated by small sigils [1]. A correspondence to Imbolc (Anagantios full moon) is not indicated.

Months

The sequence of month names of the following table assumes the calendar starts with the autumn equinox and is derived from the analysis of Monard (1999) and others.