Chamonix has two distinct peak seasons and two excellent shoulder periods. Getting the timing right depends entirely on what you want to do.
Winter (December to March) is ski season. December can be uncertain for snow, particularly at lower altitudes; January and February are the most reliable months for cold weather and good skiing. The valley is at its most animated, and prices are at their highest. The Christmas and New Year period is particularly crowded.
Spring (April to June) is a transitional period. The ski area closes progressively through April, and May is quiet with many restaurants and bars shut. But late May and June are beautiful — the alpine wildflowers are out, the hiking season is just beginning, and the town is calm. A good time for walkers who want to avoid the summer rush.
Summer (July and August) is the second peak. Walkers, cyclists, and non-skiing mountain enthusiasts fill the valley. The Aiguille du Midi cable car queues are at their worst, and accommodation prices approach winter levels. Book everything well in advance. The weather is generally warm and settled, though afternoon thunderstorms are common and conditions at altitude can deteriorate rapidly.
Autumn (September and October) is arguably the best time to visit. The crowds are gone, prices drop, the hiking trails are still open, and the larch forests turn extraordinary shades of gold against the granite peaks. The Montenvers railway and most cable cars run until mid-October.
The Ultra-Trail du Mont Blanc (UTMB), one of the world's most demanding mountain ultramarathons, takes place in late August and fills the town for a week. If trail running is your interest, this is the time to come — if it is not, this is the week to avoid.