Penn Almasy
The Holiday of Tricks and Treats gave them -Sentinel and Overseer- a war for dominance (the Treat), and then Jack stripped them of their powers as the fight ended (the Trick).
I'm assuming that they assume that using the Holiday of Giving will somehow get their powers back. Maybe Santa, oblivious to their true intentions (the Overseer was nice, he MAY get his form back since he only wanted to defend Gaia), will give them their powers back? Jack loves to see chaos, and he may not intervene (again) until it's too late to reverse it.
3nodding
this is something I've noticed too. Halloween tends to be a 'versus' holiday. There are always teams working against each other for superiority.
Christmas, on the other hand, tends to be the 'working for a greater good' holiday. There was that one year when there were alien presents vs the santa presents but you could get both. It didn't actually matter. Last year was the whole 'let's get santa back!' thing. And while the story line might have people fighting, sorta (ala Jack and Santa), the event never has us do so.