The festivals in Spain range from simple church parades to week-long street parties and folk dancing. They generally celebrate events in the church's calendar, local patron saints (e.g. Santiago de Compostela) or historical events (e.g. Moors and Christians) or harvests (e.g. La Tomatina). The variety of costumes is amazing, with every region, and indeed villages, with their own designs and colours. This is the real Spanish dancing of the people; not with the fancy Spanish flamenco-type dresses as seen in the tourist cabaret shows. A few years ago we were lucky to be in Burgos when a regional festival was on. Groups were arriving from outlying districts and, accompanied by their own musicians, danced through the town towards the cathedral. In the evening there was a massive fireworks display.
If you are so inclined, Pamplona has the bull running through the streets and many towns have weekly bullfights, with top matadors during the their festivals. I must say, I gave up going to bullfights many years ago, but, on the other hand, I did enjoy them at the time.
Unfortunately, many of the festivals are outwith the normal tourist season, so many are missed by the traveller. Check out the Spanish Tourist Board site for events and dates in your holiday region. You never know, there could be one happening in the next village to your resort.