The Celtic calendar is a compilation of pre-Christian Celtic systems of timekeeping, including the Gaulish Coligny calendar, used by Celtic countries to define the beginning and length of the day, the week, the month, the seasons, quarter days, and festivals.

The Coligny calendar is a bronze plaque with an inscribed calendar, made in Roman Gaul in the 2nd century CE. It lays out a five-year cycle of a lunisolar calendar, each year with twelve lunar months. An intercalary month is inserted before each 2.5 years. The lunar phase is tracked with exceptional precision, adjusted when necessary by a variable month, and the calendar uses the 19-year Metonic cycle to keep track of the solar year. It is the most important evidence for the reconstruction of an ancient Celtic calendar.