The tomatina is a well known festival that takes place every year in Buñol, west of Valencia.
Besides the tomatina there are a lot of festivals throughout the year that you could visit.
7-8 September: Fiestas de la Mare de Deu de la Salut in Algemesí, Valencia
24 September: Fiesta De La Mercé, Barcelona
Late-September: San Sebastián International Film Festival
12 October: Hispanic Day / Columbus Day, throughout Spain
Late-October to early-November: Annual Horror and Fantasy Film Festival, San Sebastian