I was hunting around trying to find out when the wine festival in Vilamartín on the Invierno is, and I came across what looks like a very complete listing of fiestas in Galicia. You can search by town, by type of festival, by dates, etc. And there is no shortage of fiestas in Galicia!
http://www.paxinasgalegas.es/fiestas/
http://www.paxinasgalegas.es/fiestas/