Changes

With a leap day every five years, the Coligny calendar gives the length of a solar year as 365.20 days. Since the true length is 365.2422 days, the calendar drifted by one day every 23.7 years.<ref name="Olmsted"/>

With a leap day every five years, the Coligny calendar gives the length of a solar year as 365.20 days. Since the true length is 365.2422 days, the calendar drifted by one day every 23.7 years.<ref name="Olmsted"/>

The amount of drift can be used to estimate the age of a calendar. According to Pliny, the Celtic months started on sixth day of the lunar cycle in the period 120-100 BC. As the Celtic lunar cycle drifted by one day every 199 years, this suggests that the calendar had been operating without correction for about 1,000 years.<ref name="Olmsted"/>

The amount of drift can be used to estimate the age of a calendar. According to Pliny, the Celtic months started on the sixth day of the lunar cycle in the period 120-100 BC. As the Celtic lunar cycle drifted by one day every 199 years, this suggests that the calendar had been operating without correction for about 1,000 years.<ref name="Olmsted"/>

Assuming Samhain and Beltaine transferred to the Julian calendar around AD 200, that gives us 1,300 years of solar calendar drift (1100 BC to AD 200), or 50.6 days worth of drift. November 1 is 51 days before the solstice on December 21. That this rough calculation yields the desired date exactly is surely an artifact. But placing the start of a season on the first of the month has obvious advantages in terms of convenience.

Assuming Samhain and Beltaine transferred to the Julian calendar around AD 200, that gives us 1,300 years of solar calendar drift (1100 BC to AD 200), or 50.6 days worth of drift. November 1 is 51 days before the solstice on December 21. That this rough calculation yields the desired date exactly is surely an artifact. But placing the start of a season on the first of the month has obvious advantages in terms of convenience.