Note: I know it seems out of character that Brennan would give Booth such a gift, but let's just say that even if pop culture isn't her thing, she does research.
The day after Christmas, they find human remains in a slaughterhouse and for that time both stop thinking about mistletoe and trees and Vermont.
Later, at the diner, she slides a box across the table and he raises an eyebrow.
"What happened to a holiday dedicated to self-promotion?"
"Well, if it's an exchange of goodwill and-" she catches him smirking "- I thought you knew all the rules!"
"There don't have to be rules for everything, Bones. Christmas is different."
Different strikes a chord with her, and he can't help but grin when he unwraps a pair of Grinch socks.