|From Iamanowl's website|
In the pagan world, mostly the Celtic world, Mabon is the time of the second harvest which is celebrated at the turn of the end of the summer beginning of autumn. Mabon is usually known as the son, or the great son, a Welsh deity who was saved by King Arthur -- according to some legends -- after he was abducted by Modron (the great mother) a reminiscence of Demeter (Persephone's mother) only a few days (or nights) old. That's why he is said to be a masculine counterpart of Persephone.
The first harvest being Lammas (wheat harvest) and the third Samhain better known as Hallowe'en
Later, the Christian tradition set Michaelmas on September 29, Saint Michael being the archangel who defeated Satan and continues to fight against evil. This celebration is now gone but still coincides with the Autumn term of prestigious universities in the United Kingdom.
In the USA, it is on October 1 that the US Supreme Court convene of their new term too. In Wales and England, it is one of the four legal terms too.
To go further with the National Geographic