christmas day dim sum
While I agree that Koi Palace has some of the best dim sum in the Bay Area (but have issues with Yank Sing) I think you'll find more joy in San Francisco Chinatown. The streets and restaurants will be crowded with Chinese families, with many of the kids all dressed up (including some looking uncomfortable in traditional Chinese garb). I always consider the social aspect (is there a Chinese word for Gemütlichkeit?) to be an important part of going for dim sum. Great Eastern (dim sum by order) and Gold Mountain (dim sum from carts) are two posibilities. You'll find a buzz there that you might not at YS or in the 'burbs.
I have never known a Chinese restaurant to be closed on Christmas. I think the main considerations, therefore, are:
1) Which is the best dim sum, period;
2) Which would take a reservation for your extended family since Christmas Day is busy for dim sum
Since #1 gets raised repeatedly on this board, take a look at the various opinions. Regarding #2, I have found that Hong Kong Flower Lounge, Fook Yuen, and Zen Peninsula (all in Millbrae) all take call-in numbers or reservations and stick to them pretty closely. Koi Palace, on the other hand, would probably be an enormous p-i-t-a for a large group on a crowded day.
Good yontov. See you there.
re: david kaplan
All of the good dim sum places are going to be seriously crowded on Christmas day. It might be their busiest day of the year. I would amend your statement slightly and say that I don't think there will be many authentic Chinese restaurants closed on Christmas day, but it's possible that smaller neighborhood places catering to non-Chinese diners may well be closed.