The two festivals that Sevilla is best known for are Semana Santa (Holy Week) and La Feria de Abril (April Fair). Both take place in the spring with the Feria usually beginning two weeks after Semana Santa. Unless you have a specific interest in seeing the processions during Semana Santa, it is advisable to avoid Sevilla at this time. Hotel and holiday rental prices almost double, tapas bars and restaurants are crammed full of tourists and locals and have special “Semana Santa menus”, and basically you will not get to experience the real Sevilla.
The Feria is located away from the centre of town, but visiting during this time you will often come across shops and bars that have closed for the Feria, as well as higher hotel and holiday rental prices.
For lovers of Flamenco there is a wonderful international festival every two years, the Bienal de Flamenco, which takes place in September.