I ran an extended adventure for a Bjornaer magus - part of an Initiation Quest - which began at the Gathering of Twelve Years. The two other characters took on Bjornaer characters; but rather than them playing random storyguide characters, I had them create two contrasting Bjornaer magi of a similar age to the player's character. But then I was doing more than just the Gathering: there was a story that followed.
The story really begins before the characters get to the Gathering. There is a minor encounter in getting to Crintera; most people land at Dranske and walk along the peninsula. However, the locals will know that it is the Teufelsjahr, and will be ready for the arrival of witches and demons. The characters should probably try to avoid them. There is also finding the Watching Path, and travelling up the regio with its form-changing effects. Did the character bring grogs or companions with him? If so, what animal forms do they take? Any visiting magus will probably seek out his Animal Power, and have a short communion with them (All these things are discussed under Crintera's description in GotF).
The Bjornaer chapter of HoH:MC gives a precis of the Gathering. There is an opening ceremony by the Bjornaer council which causes everyone to adopt heartbeast shape, verifying that all present are actually members of the cult. Initiation is next, conducting the Ritual of Twelve Years for all apprentices present. This can be political - some magi may desire to be present at the ritual for certain apprentices, but their presence may well interfere with the ritual.
If there is a vacant Council seat, then elections are then held. The rest of the Gathering is about honoring the memory of the ancestors. As Matt Ryan has already said, the feel is possibly more akin to a religious ceremony than a political gathering. HoH:MC suggests a few events - songs, recitations, mystery plays are performed that celebrate the life of a famous magus. There is probably a lot of trading of information; magi may be seeking a specific Great Beast who would make a suitable Mystagogue for the Inner Heartbeast, and want to trace down rumours. These might be costly secrets. Furthermore, Crintera is an Ancestor Site, so most (if not all) Clan and House Mysteries can be initiated. Even if the player's character is not seeking initiation, he might be asked to participate in another's.
There is also the matter of settling rivalries and reaffirming friendships and alliances. The former is probably couched in ritual, requiring characters to participate in trials, the purpose of which is to learn to respect another's strength, and possibly even his point of view. For example, a PC magus might meet up with his childhood bully, who was the apprentice of his master's sept-mate. This has never been a Feud or Enemy (as per the Flaw), but he is still sufficiently obnoxious to warrant a smackdown, so the PC may want to manipulate him into a trial.
There is a scheduled recitation of the news of the Order (from the Eagle Elder), news of the House (from the Horse Elder), and house policies (from the Bear Elder). All of these are open to debate and discussion.
I give attendees 5 free experience points in Bjornaer Cult Lore. These 'free' xps do not detract from seasonal experience, they simply represent who the character has talked to, and what he has learnt. (I do the same for Tribunals, granting free xp in Order of Hermes Lore). They may wish to put seasonal experience into Bjornaer Cult Lore as well, but even those who aren't trying to learn pick up something. The Gathering is too short for anything other than Exposure experience, but the character might find a Teacher at Crintera who will permit him to stay on for the rest of the season. This is contingent on him making an impression of some sorts (as a perpetrator or victim of some deed; those who were just bystanders need not apply!).
Hope this helps,
Mark
