My son walked the Frances during the month of September about 10 years ago and though he doesn't drink, he really enjoyed all the grape harvest festivals he happened upon. You are going to come across a lot of them, I'll bet.
I don't know if grape harvest festivals have the same type of programs as the annual saints' days fiestas, but if they do, you should be prepared for very late night music. I arrived in Silleda on the Sanabres a few weeks ago and saw all the bands setting up for the last night of their fiestas. After corroborating with some of the locals that this meant there would be music all night in the center of town, I decided to walk another 6.5 km to Bandeira. I am glad I did, because some pilgrims I met the next day in Ponte Ulla told me that the music did not stop till 6 am. They had not gotten a moment of sleep. Sounds like I made the right decision, even if it meant I didn't get to enjoy the festivities.