Whenever I come to a point in a story where there's heavy use of magic, I usually pause and do a dive into the ancient civilization that's inspiring the current project. Right now the book I'm working on is Exiles of Eire 3, so I went into the familiar territory of ancient Ireland.
Unfortunately since it was a culture with such a strong use of oral tradition, not much is found to show exactly what their traditions were. But what they did leave behind suggests it played a major role in their civilization. Neolithic monuments like Newgrange and Stonehenge in the British Isles that line up with major solar events demonstrates how important tracking the movements of the sun was to them. The Coligny calendar found in France showed a lunar based calendar the Gauls may have used. They also had their own names for star constellations, taken from Grecian/Roman influences.
While historical specifics can only be speculated at, it gave me plenty of fodder to play with for my fantasy interpretation of the Aos Si.