Dusseldorf is famous for its family-style brewery-restaurants, dozens of them. You can expect the host to seat you at a big table that already has some other guests, who will be glad to see you. Brewery Zum Schlüssel“ is nearby, but I haven't been there. It's only a little farther to Brewery im Füchschen, which I have been to and enjoyed. You will pass ten others on the walk.
I was disappointed with the alleged "oldest brewery" (or was it oldest restaurant?) in town, Brauerei Zum Schiffchen. It was just so-so food, and didn't have the festive atmosphere that im Füchschen did. I had the popular pork knuckle, but the crispy skin was so impenetrable that I had to turn it over and attack from the bottom.
I wasn't there at Christmas.
Edit: I forgot to say that Altbier is traditionally served in 0.3 liter little glasses. I can't say why, except that's to make it "better" than neighboring city's Kolsch, which is served in 0.2 liter glasses! I read in Lonely Planet not to ask about Kolsch in Dusseldorf; It's like wearing a Yankees cap in Boston. In Dusseldorf (if not elsewhere in Germany), Altbier is served (like Kolsch in Cologne) from gravity-fed casks set up on the counter.
The local herbal liqueur, Killepitsch, is very popular beside a beer. It's much better than, say, Jagermeister. Has a lot of dried fruit notes, rather than medicinal taste.