In 1897, a fragmented bronze calendar was discovered in Coligny, France. It is believed to date from around 50 BC and appears to be the remains of a Romanized Gaul model of a Celtic lunar and solar calendar.

It displays a cycle of about five years on 62 tables. Unlike our present day calendar, courtesy of 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 the Gaul model, the month was divided into two 15 day periods.

Now, 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, equal only 354 days. The Coligny Tablet/Calendar had 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 so that the new year started at Samhaim, see Pagan Holidays (November 1) so that it would always occur on the same day of the solar cycle