Personally, I would get a vet to see her and make sure it is just gunk and not an infection, impaction or like another poster said, sarcoid.
I would certainly not wait until January or even later this month, this needs to be taken care of now. If it is something that is not just gunk, waiting later this month or until January could be bad for your horse.
My question, WHY are you not calling the vet out now? I guess I just don't understand why people wait until the barn or others call the vet even if it is a month or so down the road instead of making a call or trailering the horse to the vet. I realize it is cheaper to split a call between more than one person, and that's fine for something simple like shots or whatever, but not a questionable lump .