January
January 17 - Fiesta de San Antonio Abad, all over Spain but in particular in Jaen, Alpujarras, Valencia and Tenerife, with partying and bonfires on the night before. In Barcelona it is celebrated on the nearest Saturday to the 17th.
February
Week before Lent - Spain's other big gay community, that of Chueca in Madrid, also goes mad for the Carnival. Andalucia is also Carnival-land, although with less of a gay input. Cadiz hosts the most famous procession. Read more about Carnival in Spain.
March
March 12 until March 19 The first truly important festival of the year - Las Fallas. Valencian families make paper mache dolls - sometimes in the shape of traditional figures, sometimes in the shape of famous people like Shrek or George W. Bush. The creations are paraded before being burnt in one of the many bonfires. This takes place amid much partying! You havent seen bonfires until youve seen the ones the Valencians light on the last night of Las Fallas.
March 23 - 30 - Mostra de Cinema Llatinoamericà de Lleida, Latin American film festival in Lleida in Catalonia.
Third Sunday of Lent - Fiesta de la Magdalena in Castellon de la Plana, Valencia, a celebration of the end of Moorish rule.
April
April 23 - La diada de Sant Jordi, in Barcelona, a type of combined Valentines Day and celebration of the deaths of Cervantes and Shakespeare (who both died on the same day).
May
No events of particular interest in May.June
Whole month Grec Performance Arts Festival
Early June Pascua Granada, a musical procession, takes place in the Barceloneta barrio of Barcelona.
Early June Marato arts festival.
Mid-month Sonar Festival of Advanced Music & Art takes place in the middle of the month, usually over three days.
June 21 Festa de la Musica
Third week of the month - Cinema Jove International Film Festival in Valencia.
July
First two weeks - Bous a la Mar (Toros en el Mar), bull run to the water's edge in Denia, near Valencia.
August
August 4 - Fiesta del Cristo de la Salud, El Palmar, Valencia (the home of Paella).
Last Wednesday in August Tomatina Tomato Fight, of course! Read my Tomatina Tomato Fight Guide for all the details, including some great photos.
September
First two weeks of the month Feria in Teruel.
September 4-8 Festival in Barbastro
September 8-14 Partying in Albarracin
September 11-16 - Jornadas de Guitarra guitar concerts in Denia, near Valencia.
September 12-15 Graus has its festival with the highlight being the Mojiganga satirical procession.
September 12-15 Festa de Santa Tecla in Tarragona, Catalonia, including Castell human tower building.
12-15 Fiesta Mayor y Corre del Bou. Cardona (Barcelona). Bull run in the streets, concerts and dancing with people dressed as giants.
15 Fiesta de la Vendimia, Ainzón, Zaragoza. The Campo de Borja wine region has its grape harvest festival, with plenty of grape stomping, starting around midday.
September 22-25 Festa de la Mercè, one Barcelona's biggest celebrations. Includes fireworks, people on stilts and, of course, copious amounts of alcohol.
October
October 4-8 - Barcelona International Erotic Film Festival.
October 5-14 Sitges, near Barcelona, has the Sitges International Film Festival of Catalonia.
October 6-15 Barcelona International Gay and Lesbian Film Festival
October 19-26 Mostra de Valencia Cinema del Mediterrani film festival in Valencia.
Last weekend - Festa de Sant Martirià in Banyoles, Catalonia.
November
November 10-18 - L'Alternativa Barcelona Independent Film Festival
December
Christmas Eve Noche Buena, a major family night. Most shops and restaurants will be closed.
Christmas Day Navidad. Things will start to open in the afternoon, though you should book in advance if you want to eat.
New Year's Eve Noche Vieja in Spanish. A family affair until midnight, when everyone gathers in front of the TV and eats 12 grapes at the stroke of midnight.

