When determining Celtic holidays, is it appropriate to use a Celtic lunar calendar or the current solar Gregorian calendar?
Are you thinking of one specific Celtic culture here, or are you trying to derive some sort of pan-Celtic calendar? Are you planning to include the Celtic regions of France, Spain, Italy and Turkey, too?
Luckily, all Celtic cultures use the Gregorian calendar now, so there’s only one possible answer.
Nadolig Llawen.
My bad…Specifically Brigantes ca. 100BC-100AD.
Which Brigantes? The tribe is attested in both Ireland and what is now England. They were evidently related tribes, but by your period one would be speaking Proto-Primitive Irish and the other Proto-Brythonic.
We don’t have direct evidence, but my best educated guess is lunar. I base that on two pieces of evidence: The Roman-period Gaulish Calendar of Coligny was both lunar and solar, and it shows Roman influence in its form. In other words, there’s no reason for the Gauls at that period to innovate a lunar calendar, and yet they evidently had one.
In Ireland, the only evidence is that the native month-names don’t always seem to correspond to the equivalent Julian months. Sometimes they do, but sometimes they seem to be about two weeks off, or (possibly) to refer to only a half-month period.
This is pretty flimsy evidence, but I think it’s the hypothesis to go with.
As far as holidays go, which ones are you using? There’s no evidence that the solstice was celebrated in the British Isles, contemporary Neo-Pagan beliefs to the contrary, but the quarter days are pretty consistent.