It was therefore a full week later - once they had surreptitiously obtained hairs from innocent Muggles who were Christmas shopping, and had practiced Apparating and Disapparating while underneath the Invisibility Cloak together - that Hermione agreed to make the journey.
Harry Potter and the Deathly Hallows
What's the grammar point for "who were Christmas shopping" then? It looks like Christmas is an adverb to modify the verb shopping, but Christmas is only a noun according to this dictionary. How should we understand the grammar here?