I stick by what I said: in Devon, the tradition (or at least the modern habit) is to hold the Wassail around the middle of January, on or close to the date of the Old Twelfth Night. Google for "Devon Wassail" if you want to see numerous examples of this.

I don't know (to answer doc Tom's other point) of a continuous tradition, though. The Wassails I've been involved in in Sussex and in Devon are both, as far as I know, revivals from about the 1970s.

Marje