In modern pagan circles, the Autumn Equinox is known as Mabon. Our favorite witch, Beth Owl’s Daughter, has a beautiful post and gorgeous imagery about this holiday :
Today is the Equinox, taken from the Latin for “equal night.” It is the Autumnal equinox in the northern hemisphere, and the Spring equinoxbelow the equator. In both cases, today consists of exactly twelve hours of daylight and twelve hours of darkness. At 5:19 this afternoon (Eastern time), the Sun will be directly above the Earth’s equator.
In some cultures, this date is known as the Witches’ Thanksgiving and many Earth-based traditions call it Mabon. Some lore says this is for Queen Mab of the Faeries, and the Celtic heroine Queen Maeve. But the name Mabon actually has links to the Mabinogion, the ancient stories of Gods and Humans in Welsh mythology. The tales of the Mabon are the “tales of the hero.” They derive this meaning from “mabon” or “meibon” — meaning a young man or youth. It is also the name of the God named Mabon ap Modron (Mabon in Welsh means “son”). So this is a reference to the son of the Welsh Goddess Madron. She is the Divine Mother and He is, simply, the Divine Son.
Most scholars agree that the Celts did not call the Autumn Equinox by the name Mabon. But this newer adaptation is certainly in harmony with the fine ancient Celtic practice of adopting festivals, myths, and Deities from other cultures.