Aiden Kelly wrote:
“Aidan Kelly Well, I did devise Mabon as the name for the fall equinox, in the Pagan-Craft calendar I pit out from 1973 to1975. It was esthetically unpleasing that there were no names for the summer solsticew and fall equinox. Finding Litha for the former was easy. Examining the Saxcalendar included in Bede, one can deduce that Litha was their name for the summer summer solstice; I later discovered that Tolkien had figured that out also.”
AND
“The fall equinox was harder, but. In one of Fred Hoyle’s books on Stonehenge, which I had the huge honor and pleasure of editing when I was with Scientific American books, in the midst of his proof that Stonehenge did and still does predict eclipses of both sun and moon, he found that a small secondary circle pinpoimted the date of the fall equinox, so it was very important thousands of years ago. Michaelmas near the equinox has long been an English festive day, perhaps because Michael had been equated with Lugh, after whom London was originally named. And given the story of Kore, it seemed logical that the parallel story of a young fertility god who was rescued from the underworld , as in the Mabinogion tale, could have been the myth associated with that date. Not certain, but also not invented arbitrarily.”

The entire discussion would be of interest to those trying to understand the continued use of this term.