If you want to live the "real Spain" you might want to come one of the following dates:
"Cultural Itinerary of Europe" should indeed choose 2004 as it will be the only chance in 7 years to see the North Gate open.
2009 - During the year: TOROS FESTIVAL
Bullfighting is the Spanish National Festivity, no doubt one of the most famous and polemical celebrations with fans and detractors all over the world. There are Fairs during the year in cities with an old bullfight tradition like Sevilla and Madrid that might be an extra attraction to the dates of your event.
2009 - February 4th to March 1st: CARNIVAL IN TENERIFE
Although we would never dare to compete against the popularity of the Carnival in Rio, the celebrations in Santa Cruz de Tenerife, in the Canary Islands, is really worthy a visit.
2009 - March 19th (same day each year): FALLAS IN VALENCIA
You might have heard about impressive beautiful cardboard figures that get burned in the night of Saint Joseph, while the roar of fireworks resounds all over the place? Well, that's Valencia in the night of nights, the highlight of the Fallas celebration.
2009 - 5th to 12th April
2010 - March 28th to April 4th: EASTER IN SEVILLA
You don't have to be a believer to enjoy the deep respect and devotion that characterizes the Eastern Processions and Celebrations in Andalusia. One of the most emotive moments is the "Madrugá", the night of Holy Thursday to Holy Friday, when nobody in the city sleeps, and from the balconies they wait for the most important Brotherhoods to walk their Saint. Simply amazing.
2009 - April 20th to May 3rd
2010 - 20th to 25th April
After the solemnity of the Eastern celebrations, the capital of the Andalusia region explodes in a 24 hours/6 days non stop party with lots of food, wine and, of course Flamenco. The festival takes place in a specific area of the city, meaning you can enjoy the normal Sevilla and, on top, have access to that frenzy mad week we call "Feria de Abril".
2009 - July 7th (same day for all years): SAN FERMINES
If Hemingway felt in love with this celebration, there must be a reason. Running in front of the bulls through the tinny little streets of an old town in Northern Spain sounds a bit risky, but somehow more than 1.000.000 visitors cannot be wrong, isn't it?
2009 - August 26th
2010 - August 25th: TOMATINA IN BUÑOL
Every year, the last Wednesday of August a tinny little town near Valencia over 30.000 pax from around the world go nuts in the Tomato party, called Tomatina, consisting basically in throwing tomatoes to any human being near you. In the last edition of the Tomatina 120.000 kg tomatoes were used. That isn't probably the cleanest activity you can offer your group, but you wouldn't deny it is quite something, eh!.
Alternatively there are some other smaller celebrations taking place during the year that you might be interested on, especially if they take place meanwhile you're around...
JANUARY
19-20 Tamborrada in San Sebastian (drums festival on the street).
FEBRUARY
Muslims & Christians Celebration in Bocairent (Valencia).
Carnival in Cadiz and Santa Cruz de Tenerife (Canary Islands).
MARCH
International rally of Old Timers in Sitges (Barcelona).
Eastern Passion in Esparreguera (Barcelona).
APRIL
Palm Sunday Parade in Elche (Alicante).
Flower Market in Vic (Barcelona).
Muslims & Christians Celebration in Alcoi (Alicante).
Saint George: the rose and the book day in Barcelona.
MAY
Festival of the Cordoba Patios (courtyards) in Cordoba.
Local celebrations in Santa Cruz de Tenerife (Canary Islands).
Horse Festival in Jerez de la Frontera (Cádiz).
Rocío Romería (celebration pilgrimage) in Almonte (Huelva).
JUNE
Sardana festival (regional dance) in Calella (Barcelona).
Corpus Christi in most Spanish towns.
Flower festival in Sitges (Barcelona).
Saint Jean and Saint Peter bonfires all over Spain.
Medieval Theatre Festival in Hita (Guadalajara).
Sport-touristy International Rally in the Noguera-Pallaresa River (Pyrenees).
Dance of Men on Stilt in Anguiano (la Rioja).
AUGUST
Octopus festival in Carballiño (Ourense).
Farmers week in Murcia.
Vintage celebration in Requena (Valencia).
Flower battle in Laredo (Cantabria).
Horse races in the beach of Sanlúcar de Barrameda (Cádiz).
SEPTEMBER
Horse and bulls celebration in Segorbe (Castellón).
Carthaginians and Romans fights in Cartagena (Murcia).
Celebration of the grapes vintage in Logroño (La Rioja).
Local festivity of Sainte Mercé in Barcelona.
OCTOBER
Celebrations of Sainte Pilar and Hispanity day (Zaragoza).
NOVEMBER
Saint Martín's Humanitary celebration in Moreda (Asturias).
DECEMBER
Christmas Flea Markets all over Spain.