We spent Christmas in Sevilla in 2002 and it was beautiful. Yes, as in most predominantly Christian countries, most everything is closed. However, we had a fabulous Christmas lunch at a very fine seafood restaurant. We also went to midnight mass at the Cathedral, which was beautiful. It was great to see the Cathedral as a part of the community. As a plus, it is not as cold in Sevilla as it is in Madrid. I'm sure your hotel or hostel can recommend open restaurants. Stroll the Parque Maria Luisa, the river and neighborhoods. You will be enchanted.