There have been a number of unanswered posts on this subject. I have been looking around for my own reservations. Here is what I found. It is really a pain that most of these places no longer advertise in the Sunday pink section of the Chron.
As far as web sites that list restaurants open on Christmas .
About.com has a list with some restaurants Ive never heard of. It mentions that Suppenkuche will be open which just strikes me as a Christmassy place to go.
Open Table is accepting reservations at the following restaurants for Christmas
Citysearch has a small list
Most of the big hotels will have brunches. I wont repeat hotels listed in the links above like the Nikko (Anzu) , Hyatt Regency (Equinox). Campton Place, Garden Court, etc
Jesters at The Argent is offering a pretty good looking holiday buffet. Ive had holiday buffets here multiple times while the hotel has gone through multiple changes of ownership and Ive never been disappointed. Havent been to the hotel under the current ownership. Heres the link. Click on the box with the Christmas tree for the menu.
Lehrs Greenhouse has the details about their holiday buffet on the link below. Haven't been there since fern bars were the rage. It is my choice this year. Festive, big selection, yet not overly fancy.
The Grand Hyatt in Union square is having a buffet for, I think, $49 at their Grandview restaurant. I went there over a decade ago, so you can figure that in. It was ok, but not my favorite. Seatings at 1:30, 4:30 and 6.
Silks at the Mandarin Oriental will have a special dinner on Christmas Eve and Christmas day. Ive always wanted to try them, but have not yet. Too fancy for the crowd Im dining with this year. Ive always loved their food, but it has been about two years since I ate there and they had just changed chefs.
There will be a buffet brunch in the Crown Room from 10-6 for $85 at the Fairmont. The Tonga Room will have a 4 course prix fix dinner where turkey will be available. I had the buffet at the Crown Room at the Fairmont, again, over a decade ago. It was very, very fancy and there is a view. I seem to remember lots of jellied, jewel like appetizers.
Asia de Cuba at the Clift will be having a special pre-fixe dinner from 5-9 for $59. Last time I ate here on a holiday was prior to Asia de Cuba.
Dont know if the Oak Room will be open at the St Francis. The buffet I had there, again, a decade ago was unimpressive. Michael Minna will be closed.
There is a three course prix fixe dinner for $75 at the Four Seasons. Theres a vegetarian option available.
Even though the Ritz is listed on open table, they have a description of what is available on the hotel website, so Im including the link. I had the champagne brunch there two years ago. I was going for super fancy, I prefer the Fairmont. The service IS wonderful though and the hotel lovely.
Heres my Ritz Experience
[BROKEN LINK REMOVED]
The Cliff House is in several lists. I would advise against going there. Heres a link to my Thanksgiving experience there this year.
There are probably more, but this is all I came accross when looking for myself.