Last November 1, I attended the 11:45 am High Mass for the Feast of All Saints, arrived about an hour early got one of the last seats in the transcept. Although not as big as the Feast of St James, it was a still the celebration of a major feast day, with the Archbishop, numerous priests, a procession of the Saint accompanied by bassoons and oboes, a beautiful choir, and the Botafumerio, used as part of the Mass, and not just stuck at the end. Some of the seats up front were reserved, but not all. And they welcomed pilgrims at the beginning, as they do at the Pilgrims Masses. It was really beautiful and definitely worth the waiting time beforehand. If you want to attend the Mass for St James, go for it. Just show up really early. Good luck and Buen Camino