The Celtic calendar has been estimated by scholars based (amongst other evidence) on the re-assembled remains of a 5ft wide, beaten copper, ‘perpetual’ calendar buried in the ground in Coligny, Gaul, to protect it after it was destroyed, probably by the Romans.
The Coligny calendar had little peg holes next to every single day which helped the Gaulish Celts track time across a fixed five year (60 month) period. It established a year of twelve months of 29 or 30 days with a leap month every 2.5 years to correct seasonal drift. While it seems fiddly to interject a whole month every 2.5 years it’s much simpler to think of it as every thirty months. Simpler still if you consider it as mid-way and end of the 60 month period. Suddenly it’s quite straight forward and must have made as much easy sense to the Celts as “thirty days has September, April, June, and November…” does to us.
To stop it slowly creeping too far the other way, every five ‘sets’ of 60 months (ie: every 30 Coligny years or 40 actual years) they would finish the period and NOT have the extra month, dragging it back by 30ish days. Those simple rules kept the Coligny Calendar (ie: the Celtic calendar) ticking along accurately and comparisons to the modern/Gregorian calendar certainly show it keeps admirable pace with our own sense of timekeeping.