Will all shops, restaurants, bars & sights be closed? Should we bring groceries for those days? And they open again on the 26th?
In Spain the biggest family occasion at Christmas is the evening of the 24th December. Most families eat together.
Consequently after about 6pm all the shops close and almost everything closes. It is very difficult to get a taxi, almost all restaurants are closed. At about 11pm people start going out and bars start to open.
On the 25th of December almost everything is closed but there are more things open than on the 24th.
The 26th is a normal day.
It is a good idea to go shopping before 2pm on the 24th of December and buy enough provisions to last at least until the 26th.
Have a look at this page which has some Spanish vocabulary and some other notes about Christmas in Spain.