Barcelona is very quiet those two days. Nearly all businesses are closed including restaurants and tourist traps. You may be able to get a reservation at a few places for the 24th but it will be pricey and need to book in advance. Your concierge can probably set something up. Most places will be closed, though we are open both days in Sitges (I think we will be the only ones).
From what I can tell you should be able to get to Montserrat on the 25th, they will be saying mass etc. up there and the train is running on a reduced schedule.
By the evening of the 25th things will be coming back to life and you can find some bars etc open.