Yes, it is the national holiday on Oct 12. Two years ago my husband walked into Foncebaden and got one of the last beds in town at the donativo on Oct 12. He tried all other albergues first, but they were full. Many tour buses, etc. Many Spaniards on holiday!
You may also see before that the celebration of San Froilan in the Leon and Lugo areas late Sept to Oct 10. Around Leon lots of fairs and street activities.