Thun is a year-round destination, but the seasons make a significant difference. Summer (June to August) is when the lake is at its best for swimming and boat trips, the Thunerseespiele runs its open-air shows, and the mountain views are clear. The Bernese Oberland has its best hiking weather from late June through September.
Spring is excellent: the snowmelt feeds the Aare, the castle gardens and Schadau park are in flower, and crowds are thin. May is arguably the best month — long days, mild temperatures, and the old town nearly to yourself.
Winter in Thun is mild by Swiss standards — the lake moderates temperatures and heavy snowfall in the town itself is uncommon. The Christmas market on the Rathausplatz is one of the better ones in the region. Ski resorts in the Bernese Oberland are a short train journey away.