I think that some of the difficulty you may be encountering could be down to the fact that the major festivals of the Celtic cultures of which the records have survived weren’t on the solstice or equinox dates, so the traditional materials or literature that have come down to us won’t be that helpful. It’s also difficult because there are quite a lot of differences between the Gaelic and the British traditions, as pointed out before and the traditional materials seem to indicate that customs were highly localised and by no means pan-Celtic. It might be a fruitless task trying to standardise something that was never standardised, so to speak. Another thing is that the solstices and equinoxes are basically marking or tracing the progression of the same thing throughout the year, i.e. the sun. So, in theory, the four solar festivals are all connected with ideas about the sun and not necessarily with different “entities” as such. You need to take into account the differences in calendars too, our calendar shifted traditional dates by about two weeks and the ancient Celts seemed to have used a lunar calendar with solar calculations.