All Seated on the Ground isn't included, but that's okay--you can read it for free at Asimov's Science Fiction. In it, Willis considers that age-old question: what happens when you add aliens to Christmas choirs and stir? The narrator of the story is one of Willis's typical 'almost only sane person in an insane world', but it's a formula that's a whole lot of fun to read, so that's all right.
Also, not at all Christmas-related, but Everview is the YA vampire romance story I didn't know I was looking for.