It's our tradition.
Rather than ripping open and hurriedly reading the messages each evening in the hectic pre-Christmas days, we save up the cards and holiday letters. At 12th Night (or, this year, 14th Brunch) we sit down together and leisurely savor the photos, news, and good wishes of our family and friends.
We pass around the pictures of friends BBQing last summer, the envelopes closed with sealing wax whose stamps have the same image as the enclosed card, and the updates on moves, retirements, and new careers.
Very civilized. Very fun.