Is there a special meal for Christmas day? Are we able to book ahead?
I was there last month, I am sure they will have Christmas Eve dinner and Christmas dinner event, you can book ahead/advance to the hotel.
I think you might have 2 options to be able to book dining which was a small restaurant managing with bakery. not easy to book for the shortage of seats. The other is buffet restaurant on the 1st lobby. both are not big size one so not sure you will satisfy.
Get an answer from Hyatt Regency Kyoto staff and past guests.