Tamborrada
When:
January 20 every year. 
Every January 20th, San Sebastian celebrates its patron saint's day with the Tamborrada, a drum parade. Events kick off at 12 o'clock midnight on the 19th in the Plaza de la Constitución, when the city flag is raised. The drumming opens with the Sociedad Gaztelubide (an Eating Society) and representatives of other drum outfits who play to melodies by Sarriegui, a Basque composer, and continues throughout the following 24 hours.
Children, youngsters, adults and seniors either gather round or dress up as French Soldiers or cooks to drum away and march in an orderly manner through town, and gather round at MIdnight in the Plaza de la Constitución, to play the song one last time. The celebration comes to a close at midnight, when people congregate at the Konstituzio Plaza and the city flag is simultaneously lowered at various locations.  

 

Santo Tomas
When
: December 21st
The famous celebrations for Saint Thomas's Day take place every December 21st. The city turns into an enormous country market of which the absolute star is the txistorra (fresh chorizo-like sausage). Stalls are set up all over the city, offering all sorts of food and craft wares, mostly produced by the people from the local farmhouses. These include cheese, honey, txistorra sausage, cider and cakes.
The Santo Tomás celebrations date back to the mid-19th century, when the inhabitants of the farmhouses (baserritarras) in the province would make their way into town to pay their yearly rent to the owners of the land they worked. Making the most of their visit to the city, they would sell their agricultural produce and stock up with the things they couldn’t obtain locally. In time a fair took shape in the Plaza de la Constitución.
http://www.sansebastianturismo.com/en/thematic/tourism/963-saint-thomas-s-day


Regattas
When
: first two sundays of September.
San Sebastian’s regattas have been are celebrated since 1879 as part of the city's festivities. The best days to come and watch the competition are the first two sundays of September in La Concha Bay. Teams from all over the Cantabrian sea come and compete to win the most prestigious flag in the regatta season. 

International Jazz Festival
When:
during the second fortnight in July
During the Jazz Festival San Sebastián taps its feet to the rhythm of the finest jazz. Great musicians like James Brown, Liza Minnelli, Bobby McFerrin, Diana Krall, Cassandra Wilson and B.B. King have all taken to the city’s beaches, theatres, squares and auditoriums at one time or another to delight audiences at this world-famous event. 
http://heinekenjazzaldia.com/en/

 

Gastronomika, San Sebastians food Congress
When:
within the first week of October every year
One of the biggest and most important food congresses in the world, it’s up there with the 10 best. San Sebastian is known for its world-class cuisine, so it makes perfect sense that one of the most dynamic culinary events takes place in the Spanish city. Each year there’s an invited country which showcases many of its chefs, products and projects, a must-do for foodies and it holds a place among the travel recommendations made by Forbes Travel Guide. http://www.sansebastiangastronomika.com/en/


Zinemaldi Film Festival
When: Second half of September every year
The San Sebastián International Film Festival (Spanish: Festival de San Sebastián; Basque: Donostia Zinemaldia) is an annual FIAPF A category film festival held in the Spanish city of Donostia-San Sebastián in September, in the Basque Country.Since its creation in 1953 it has established itself as one of the most important cinema festivals in the world, being one of the 14 "A" category competitive festivals accredited by the FIAPF. It has hosted several important events of the history of cinema, such as the international premiers of Vertigo, by Alfred Hitchcock (who attended the Festival) or Melinda and Melinda by Woody Allen and the European premier of Star Wars. Actors and directors such as Bette Davis, Gloria Swanson, Gregory Peck, Glenn Ford, Elizabeth Taylor, Audrey Hepburn, Charlton Heston, Robert De Niro, Meryl Streep, Richard Gere, Michael Douglas, Catherine Zeta-Jones, Mel Gibson, Demi Moore, Naomi Watts and Brad Pitt have attended the festival since its inception. It was the first festival attended byRoman Polanski and it was a key element to advance the professional careers of Francis Ford Coppola or Pedro Almodóvar, for instance. http://www.sansebastianfestival.com/in/