Aste Nagusia ("Big Week," Semana Grande in Spanish) is San Sebastián's biggest annual festival, anchored around the feast of the Assumption in mid-August. It opens with the txupinazo — a rocket launch from Alderdi Eder that formally kicks things off — and from there runs an international fireworks competition: a different country's team lights up the bay over La Concha every night around 10:45pm, all week.
Around that spine, the city fills with txosnas — temporary, society-run food-and-drink booths — plus concerts, parades with giants and big-headed figures (comparsas), and rowing regattas in the bay. It's loud, social, and very easy to do alone: you're never the only person standing at a counter or claiming a patch of sand for the fireworks.
You land this evening, before anything opens. The city is normal — a good night to get oriented with an easy pintxos crawl in the Parte Vieja before the crowds arrive tomorrow.
The rocket launch from Alderdi Eder (~7pm) opens the festival. This is the single busiest night of your stay — txosnas open in Sagüés, the old town fills up fast, and the first fireworks go off ~10:45pm over La Concha. Claim your spot for fireworks earlier than you think you need to.
Your big lunch (1:30pm, booked) anchors the day. Festival energy continues but a Sunday is typically a notch calmer than the opening Saturday — an easy night to ease back in after a long lunch, with fireworks again ~10:45pm.
Weekday crowds are lighter than the weekend. Good day for markets, a nap, photography — whatever pace you want — with the festival still running in the background all evening.
Day trip to Getaria (Elkano lunch), then back to San Sebastián for your final evening — last pintxos crawl, last fireworks, before heading to Rioja the next morning.
Where the nightly fireworks competition fires from and where the biggest crowds gather to watch — also Ondarreta, slightly less packed.
Where the official txosnas are set up — the real heart of the festival's drinking-and-music scene, often louder and later than the fireworks crowd. You're staying right near here, in Gros.
The old town's pintxos density goes up a notch all week — busier bars, longer waits, but the same walk-up-and-order rhythm.
The comparsas (giants and big-headed figures) parade through here during the week — keep an eye out if you're walking around Gros midday.