How to write a love of christmas sonnet?

Choose a sonnet form, usually Shakespearian, but sometimes Petrarchan. If this were more serious poetry, you'd probably have room to goof around with the form, but for such a light subject you probably should stick to it pretty firmly. Think of a sentiment that you can comment on in a clever way, like a joke that has a punchline, and make that your final couplet (if it's Shakespearian).