Count on everything but church being closed on Christmas day — all stores, museums, cafes, public transit and perhaps even the front desk at hotels and hostels. Figure out what/where you’re going to eat and how you’ll get around in advance.

I was just in Barcelona for Christmas and as long as you stay around Las Ramblas you’ll be able to find cafes & restaurants that are open. The vast majority of tourist attractions are are either closed or have reduced hours on Christmas Eve and are closed for Christmas, but walk around the gothic quarter and you’ll find things to see/do. On Christmas day the metro runs and both Sagrada Familia and Parc Guell are open so it’s a good day to visit those two sights.