Many neopagans today call the Autumn Equinox festival by the name “Mabon”. This name was first suggested in an article by Aidan Kelly in the middle 1970s, along with Litha and Ostara to repaganize the names of the festivals. While Litha and Ostara at least have some historical and mythological connections to the Summer solstice and vernal equinox, Mabon has neither.
Mabon ap Modron is a prominent figure from Welsh literature and mythology, the son of Modron and a member of Arthur’s war band. Both he and his mother were likely deities in origin, descending from a divine mother–son pair. His name is related to the Romano-British god Maponos, whose name means “divine son”; Modron, in turn, is likely related to the Gaulish goddess Dea Matrona. He is often equated with the Demetian hero Pryderi fab Pwyll, and may be associated with the minor Arthurian character Mabon fab Mellt.
One of the earliest direct reference to Mabon can be found in the tenth century poem Pa Gur, in which Arthur recounts the feats and achievements of his knights so as to gain entrance to a fortress guarded by Glewlwyd Gafaelfawr, the eponymous porter.
In another Arthurian tale, Arthur and his men learn that Mabon was stolen from his mother’s arms when he was three nights old, and question the world’s oldest and wisest animals about his whereabouts, until they are led to the salmon of Llyn Llyw, the oldest animal of them all. The enormous salmon carries Arthur’s men Cei and Bedwyr downstream to Mabon’s prison in Gloucester; they hear him through the walls, singing a lamentation for his fate. The rest of Arthur’s men launch an assault on the front of the prison, while Cei and Bedwyr sneak in the back and rescue Mabon.
None of the above, however, establishes any connection to the Harvest with Mabon. While Mabon may have originated as a Divine Son of the Mother, nothing in his mythos associates him with any of the harvests. That he was carried off from his mother at three days old, however, does potentially link him with Taliesin of the Radiant Brow, who has a very strong Beltane/May Day association.
Traditionally, this festival was called Harvest Home or Ingathering. Per the Encyclopedia Britannica, it is a “traditional English harvest festival, celebrated from antiquity and surviving to modern times in isolated regions. Participants celebrate the last day of harvest in late September by singing, shouting, and decorating the village with boughs. The Cailleac, or last sheaf of corn (grain), which represents the spirit of the field, is made into a harvest doll and drenched with water as a rain charm. This sheaf is saved until spring planting.”
The ancient festival also included the symbolic murder of the grain spirit, as well as rites for expelling the devil, and as such has similarities to some of the Cainite lore.
A similar festival was traditionally held in parts of Ireland, Scotland, and northern Europe.