I live in Barcelona and here it's the way described in this answer. Quite often it's not that universities do not have lectures on Fridays (at least it was not the case of my university), it's that an important amount of students are not from Barcelona, so they come back to home once lessons are finished on Friday. So, undergraduate students prefer Thursday as a party day.

I've the suspicion that time slot they give you will be only important for the visit to Camera degli Sposi. We'll see once there.