After an hour or two's trawling through books last night I'm afraid I'm not significantly wiser.
The best way to answer your question is probably with another, that is 'what do you want to know for?'.

I think Cian is most likely correct about the moon being the basis for the festival date and Clive Ruggles (Astronomy in Prehistoric Britain and Ireland) gives perhaps the best synthesis in dismissing day counting (as I proposed) in the formation of a possible Celtic Calendar in favour of a combination of solar and lunar observation.
However the question of the relevance of a Celtic festival to the alignment of a prehistoric monument such as the circle at Lough Gur for example (if this is what you are wondering about?) would remain. Somerville and Thom established a number of alignments to the sun at the mid quarter point between autumnal equinox and winter solstice, a date which was proposed to correspond to the ancient festival. If I was looking at one of these sites that is probably the date I would be looking at.