Every year, Dumbledore get a decent amount of Christmas presents. New quills, expensive trinkets, old leather-bound books, he's sure he's got several viles of liquid luck stocked up from last year. So he wasn't particularly surprised when an unfamiliar hawk owl swooped in and dropped off a parcel on his desk, then swooping out again.
He was very pleased to find out that it contained a rather interesting Muggle Christmas card, along with a pair of very thick, warm, fluffy socks.