There are a number of monthly English tunes in the Lewes area populated by the same bunch of usual suspects. The code is that there will be a nominated leader who will led the first couple of sets of tunes or tune then go around the room inviting that musician to lead the next one.
One of the strange unwritten, undiscussed rules - there are several of them - is that when we reach the month of December, if anyone nominates the 3-part Heel & Toe Polka then when we play it, the usual third part will be replaced by Jingle Bells. This has certainly been going on for at least a decade, probably longer. How and when this started, I have no idea.