The Easter Feria, or festival, in Sevilla, is a three-day long celebration with special bulfights, street parties and the most enormous festival of flamenco dancing in the evenings. The local park becomes acres upon acres of tents and bars hosting individual parties attended by musicians and the women, dressed in full flamenco regalia, are a magnificent sight. I think it's in Easter. Watch out, most hotels are more expensive around this time, but it is worth it.