Lauterbrunnen is a year-round destination, though the experience varies significantly by season.
Summer (June–September) is the main walking and sightseeing season. The waterfalls are at their fullest in June and July when snowmelt is running. Staubbach and Trümmelbach Falls are most dramatic then. The valley fills with hikers and the car-free villages of Wengen and Mürren are busy but not overwhelmingly so. Book accommodation in advance, especially in July and August.
Winter (December–March) brings skiing on both sides of the valley. Wengen has the famous Lauberhorn World Cup race in mid-January, which draws 30,000 spectators. The Schilthorn ski area above Mürren is quieter. The Jungfraujoch is accessible year-round; winter visits mean snow on the glacier and lower crowds, though clear days are less predictable.
Shoulder seasons (April–May, October) are cheaper and quieter. The waterfalls taper off in autumn, and some of the higher cable cars close in November and April for maintenance. But the valley itself is beautiful in all seasons, and the light in October on the mountain walls is exceptional.
One practical note: the Jungfraujoch train is expensive (around CHF 210 for a return from Interlaken) and not worth it in cloud. Check the summit webcam before booking.