The Gaulish calendar was a lunisolar calendar, upon which the Celtic calendar I have devised is based. Not much is known about this calendar, as it was suppressed by the Romans in the first century CE, who insisted that all their subjects use the Julian calendar. The Gauls, therefore, broke up a large, bronze tablet that they used to keep track of the days in their lunisolar calendar, and buried it, perhaps so that it would not be discovered, or perhaps so that it would survive to be discovered again in the future.
The remains of the tablet were indeed discovered, over 18 centuries later near the French town of Coligny in 1897. All that we know of the calendar is contained on the tablet, and much of it is missing. Therefore, much about it has to be surmised from our knowledge of the conventional use of such calendars at around the time of its use.
The Gaulish calendar was, however, rather different in that it followed a set pattern rather than the exact phases of the moon, and intercalary months were added at set intervals rather than when needed. This was the result of much knowledge of the moon’s cycles, and the wish to devise a calendar that would be predictable into the future, and yet reflect as closely as possible the phases of the moon. It also enable them to encapsulate the calendar in the bronze tablet, and use it on a perpetual basis from year to year.
The system that the Gauls devised, was a very succinct pattern of five year blocks. In each block, they knew that there were 62 months, 60 ordinary moths and two intercalary months. They decided that each five-year cycle would begin with an intercalary month, which as far as we know was called Ciallos. The third year of each cycle would also have an intercalary month, but inserted between months 6 and 7. This cycle would be repeated for 30 years, except that in the sixth cycle, only one extra month would be added instead of two. In addition to the intercalary months, an extra day was needed every five years, so one month got an extra day in the fourth year of every cycle.The months were of 29 or 30 days each, roughly alternating between these lengths. The names and lengths of these months were as follows: