The autumn equinox marks the day between the Lughnasadh and the Samhain where the day and the night have the same duration. It is essentially a seasonal feast with astronomical and pastoral cults. It was the harvesting time, a period of abundance, but also a period to start to gather the provisions for winter. This is a feast to share with the community, to thank to the deities and spirits of the Land and to gather means to survive the winter.
It is important to notice the importance of the burials in these Celtic cults, which can bring a new light to our understanding of this day. The Sliabh na Caillí, or the “Hill of the Hag”, in Loughcrew County Meath, and Western passage of Knowth (Cnoghba), one of the three Boyne Valley passage-tombs are burial sites astronomical aligned with the autumn equinox, and at this date the rising sun enters in the passage of the tombs illuminating their several carvings. These burial mounds and their archeological findings are the proof that this feast was not only of a pastoral meaning but also related with much more spiritual cults, related with the Ancestors and the otherworld.
If we did not know much about the original meanings of this feast to the Celts, it is probably because it was not a day of political assemblies, in fact, is very plausible that there were two kinds of rituals being made at this day, one of the productive class, which led us some evidences of agrarian cults, and one of the Druids, which would have disappeared with the end of this sacerdotal class. I think that the warrior class would probably attend to the first one too, because of the lack of information about the second.
Thus, I believe that the autumn equinox is a time of connection with the Ancestors and with the otherworld, it is a time of spiritual introspection, the time before Samhain which we must start to prepare ourselves for the end of the annual cycle and for the beginning of the cold part of the year.