Another transportation option to get to Sarria on Sunday, July 21st. Drive from San Sebastián to Vitoria/Gasteiz (about 100 Km/1 hour's drive). In Vitoria/Gasteiz take the train (14:08) to Monforte de Lemos (20:21):
http://www.renfe.com/EN/viajeros/horarios.html
Then (22:05) take Monbus to...