I started looking for Santa, but stumbled on this old thread: http://www.jimbutcheronline.com/bb/index.php/topic,21066.15.html
I've been toying with Oberon in my own game, but wasn't sure where to go with him. A few of the comments in here give me a good idea though. Suppose Oberon, shown at various times to be wed to both Mab and Titania (unless I'm mistaken, if I am I apologize to the Bard fans), is in fact the King of both Spring and Fall. He owns neither season wholly, and is not a tri-part being like the Mother/Queen/Lady division. Instead he shares himself between his Queens, listens to the advice of the Mothers as he will, and at one time was father to the Ladies (who's to say he's not Lily's dad?)
This would be a slight reversal on the Persephone myth as well, or could be, depending on how strong or weak Oby is.
The vernal and autumnal equinoxes then would be his chosen time, perhaps when he goes wandering for a while and then finds himself in the court of his other Queen. His Knights, if he has any, would be female, to balance the male knights who owe year round fealty to his Queens.
All this is how it was, only one year Oberon went wandering and never showed up at either court ever again. His disposition is a mystery, and without his power present his knights eventually died, their power being subsumed by the queens (the spring knight returning to Titania, the Autumn knight returning to Mab). They fey who called themselves his have since become Summer or Winter fae, or walked away and become wyldfae, only joining in when the Call is given. In his absence each queen blames the other for their beloved's loss, even still.
Vernal magic would feel like summer magic, but have a hint of winter's ice in it, and vice versa for Autumn magic, I think, and for either knight to come back fully they'd need to seperate the power from Mab and Titania... perhaps most easily done by killing either Summor or Winter knight and taking their power.
Is it possible the Erlking is Oberon? Yes, maybe even more than likely. As the Erlking said
in Changes he no longer engages in niceties.
Edit: This thread also deals with autumn and fall courts:
http://www.jimbutcheronline.com/bb/index.php/topic,17813.0.html