who picks out the holiday movies and who makes the hot cocoa: Allison picks out movies, Jackson makes hot cocoa.
who starts the snowball fight: Jackson.
who drags the other under the mistletoe: Jackson, though he normally likes to build little contraptions so when Ally thinks she's picking a book off the shelf, she's really triggering a series of mechanics that will involve in a mistletoe dropping over her head.
who decorates the house: Both. Though Allison prefers to deal with the lighting, and Jackson builds scenes out of the snow if there's enough of it.
who hangs up the ornaments on the tree: Allison does the lights, Jackson does the ornaments.
who cooks christmas dinner: Allison, but each year Jackson gives her a new cookbook to use.
who invites the other to sing a christmas duet: Jackson invites Ally.
if they have any holiday traditions: They don't. It's just a few days of relaxation.
who would start a food fight during baking: Jackson, most likely.
who would get drunk off of eggnog: Jackson.