Dating back to the 2nd Century CE, The Gaulish Coligny calendar is the oldest discovered Celtic solar-lunar ritual calendar. However, the astrological format of the Coligny calendar speaks to a sophistication, particularly focused on a careful significance placed upon the cycles of the moon, that may represent a more complicated system that the Julian calendar was being melded with during it's imposed introduction into Roman Gaul.

The Coligny Calendar is an attempt to reconcile the cycles of the moon and sun, as is the modern Gregorian calendar. However, the Coligny calendar considers the phases of the moon to be important, and each month always begins with the same moon phase. The calendar uses a mathematical arrangement to keep a normal 12 month calendar in sync with the moon and keeps the whole system in sync by adding an intercalary month every 2+1⁄2 years. The Coligny calendar registers a five-year cycle of 62 lunar months, divided into a "bright" and a "dark" fortnight (or half a moon cycle) each. The months were possibly taken to begin on the new moon, and a 13th intercalary month was added every two and a half years to align the lunations with the solar year.

The astronomical format of the calendar year that the Coligny calendar represents may well be far older, as calendars are usually even more conservative than rites and cults. The date of its inception is unknown, but correspondences of Insular Celtic and Continental Celtic calendars suggest that some early form may date to Proto-Celtic times, roughly 800 B.C. The Coligny calendar achieves a complex synchronisation of the solar and lunar months. Whether it does this for philosophical or practical reasons, it points to a considerable degree of sophistication.

What do we know about this mysterious lunar system that may have existed before the introduction of the Julian calendar? If we have no surviving evidence, are there other calendar systems that we can look to to get an idea of how it may have operated?