The Coligny Calendar, which was found in 1897 in Coligny, Ain, was engraved on a bronze tablet, preserved in 73 fragments, that originally was 1.48 m wide and 0.9 m high (Lambert p.111). 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 Gallic language. The restored tablet contains sixteen vertical columns, with sixty-two 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.
There were four major festivals in the Gallic Calendar: "Imbolc" on 1 February, possibly linked to the lactation of the ewes and sacred to the Irish Goddess Brigid. "Beltaine" on 1 May, connected to fertility and warmth, possibly linked to the Sun God Belenos. "Lúnasa" on 1 August, connected with the harvest and associated with the God Lugh. And finally "Samhain" on 1 November, possibly the start of the year. Two of these festivals, Beltaine and Lúnasa are shown on the Coligny Calendar by sigils, and it is not too much of a stretch of the imagination to match the first month on the Calendar (Samonios) to Samhain. Imbolc does not seem to be shown at all however.
The Celtic Calendar seems to be based on astronomy but how any astrology system would have worked is harder to tell. We have to base our knowledge on Old Irish manuscripts, none of which have been published or fully translated. It seems to have been based on an indigenous Irish symbol system, and not that of any of the more commonly-known astrological systems such as Western, Chinese or Vedic astrology.