Best known among Spain's folkloristic traditions are certainly Flamenco and bullfights. You will find bullfights indeed throughout the country, the most popular event perhaps being the "Running of Bulls" during the Sanfermines in Pamplona. But bullfights are part and parcel of any Fiesta.

Flamenco, on the other hand, is the musical tradition in the country's south, in particular in Andalucia. That is where you have to move to see and listen first rate dancing and guitar playing. In April takes place Feria de Abril, in Seville, an entire week of singing and dancing and trying that great Sherry wine, those delicious tasty snacks called "Tapas" and more Sherry wine ... and if you like this sort of things you REALLY should not miss that event.
If you can arrange it (and find a hotel room) better arrive to Seville already two weeks before Feria, and see this city's other outstanding event: Semana Santa, Easter week, with its world-famous processions.
Another most popular event of religious origin is El Rocio, a traditional pilgimship to the village El Rocio in the province of Huelva, in May. This pilgrimship anyhow is not too solemn and serious, Flamenco and wine being indispensable components. Follow the thousands who each year travel there and have yourself a ball.

Another outstanding Fiesta are "Las Fallas de San José" in Valencia, in March, when all the city becomes the scenery of an enormous party with lots of good mood and excellent fireworks. Next to Valencia there is the little town Buñol which hosts each year on the last Wednesday of August La Tomatina, the world's largest tomato fight. It is unclear why the town's inhabitants started over 50 years ago the tradition of throwing mature tomatoes at each other, but today the event attracts thousands of visitors from all over the world.

A wild week can also be spent in San Sebastian during February, when La Tamburrada takes place.
Madrid has its Fiesta, "San Isidro", in May. If you like bullfights, don't miss it.

Carnival is popular in all the country, the best is in Santa Cruz de Tenerife, but if you are at the peninsula, Cadiz and Sitges are the places to go.