This form was invented by Dante Alighieri, probably for the Divine Comedy.
It uses a chain rhyme of a/b/a b/c/b c/d/c etc; and, in English, it's usually
written in iambic pentametre, as I've done above. I'm very much a beginner
at this kind of form, but it's a challenge, and quite enjoyable to have a go at.
The easy pitfall is the overuse of modifiers as an easy means to fill the metre.
I used rather a lot here, although they are fairly appropriate in this sort of
context. I also used a lot of internal rhymes, alliteration and assonance to
try and create some mood and symmetry, which seemed appropriate to the
context. It's rather more about the pagan associations of Yule than it is about
Christmas, though there are overlaps, of course - the latter having borrowed
much from the former. The symbolic associations with Yule that I've used
here are to do with fire, yellow, wheel etc. The title is also a link to a
Wikipedia article about Yule, if you're interested. For more on terza rima,
click the title of this note.