Changes

''Sanas Cormaic'', an Irish glossary written around 900, gives ''Imbol'' (February 1) as the first day of spring, ''Beltaine'' (May 1) the first day of summer, ''Lugnasad'' (August 1) the first day of fall, and ''Samhain'' (November 1) the first day of winter.<ref name="Olmsted">Olmsted, Garrett (2009). ''The Coligny Calendar: the Implications for Gaulish and Irish Festival Dates''.</ref> The Irish adopted the Julian calendar around AD 450. The seasonal dates may have transferred from the Celtic calendar at this time.

''Sanas Cormaic'', an Irish glossary written around 900, gives ''Imbol'' (February 1) as the first day of spring, ''Beltaine'' (May 1) the first day of summer, ''Lugnasad'' (August 1) the first day of fall, and ''Samhain'' (November 1) the first day of winter.<ref name="Olmsted">Olmsted, Garrett (2009). ''The Coligny Calendar: the Implications for Gaulish and Irish Festival Dates''.</ref> The Irish adopted the Julian calendar around AD 450. The seasonal dates may have transferred from the Celtic calendar at this time.

A Celtic lunisolar calendar has been reconstructed from fragments of two first century BC calendars, one found in the French town of Coligny and the other in Villards d'Heria. The first month on this calendar is called ''Samonios''. This month is cognate with Samhain. The Celts celebrated a three-day festival that began on the 17th of ''Samonios'', according to the calendar. This date may correspond to the winter solstice, which is on December 21.<ref name="Olmsted"/>

A Celtic lunisolar calendar has been reconstructed from fragments of two first century BC calendars, one found in the French towns of Coligny and Villards d'Heria. The first month on this calendar is called ''Samonios''. This month is cognate with Samhain. The Celts celebrated a three-day festival that began on the 17th of ''Samonios'', according to the calendar. This date may correspond to the winter solstice, which is on December 21.<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"/><ref>The amount of drift can be used to estimate the age of the calendar. For Samhain to move from the December-January period to November suggests that the calendar was in use for about 1,300 years. Pliny writes that Celtic months started on sixth day of the lunar cycle. This suggests that the calendar had already accumulated a thousand years worth of drift in his time, according to Olmsted.</ref>

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"/><ref>The amount of drift can be used to estimate the age of the calendar. For Samhain to move from the December-January period to November suggests that the calendar was in use for about 1,300 years. Pliny writes that Celtic months started on sixth day of the lunar cycle. This suggests that the calendar had already accumulated a thousand years worth of drift in his time, according to Olmsted.</ref>