Mabon is NOT an alternative name for the Autumn Equinox. Mabon is a character, maybe a God, from Welsh mythology. His name is Mabon Ap Modron, which literally translates as “The Son of the Mother” – his name being “Son” and that of his mother, being “Mother” to make it clear. Mabon is described amongst other things as a Wizard and as “the oldest of men or beasts living on earth”. It is also said that his story was unknown, probably tying in with the idea that he was the oldest of men or beasts, therefore how could it be known! (see http://sorita.co.uk/what-or-rather-who-is-mabon for more)
Then came along an American author who named the festival after this God, based on misinformation and a misunderstanding, and because it sounded authentic and because everyone decided to follow, rather than question, it soon became the name for the festival. Strangely, the majority of those using the name did not, and still does not seem to know, that it is the name of a God / male hero from Welsh myth.