Winter (December–April) is the main season for skiing and the period when Courmayeur is at its most energetic and most crowded. Snow cover at altitude is generally reliable from mid-December, and the upper ski area often stays open into April. The Christmas–New Year period and the Italian school holidays in February are extremely busy; if you want the slopes and the village at a manageable level, January and the first half of March are the sweet spots.
Summer (July–September) is increasingly popular and the weather is reliable. This is the time for hiking the Tour du Mont Blanc, cycling the valley roads, and using the Skyway Monte Bianco cable car without a queue. The Alpine Botanical Garden Saussurea is only open in summer. Temperatures at altitude are cool even in August, and afternoon thunderstorms are common — start mountain walks early.
Spring and autumn are shoulder seasons when many facilities close. The ski lifts typically shut in April; the summer hiking season doesn't fully open until late June when snow clears from the higher passes. October–November sees the resort largely closed. If you are visiting off-season, check what is operating before you go.
The Courmayeur Noir Festival, a literary festival focused on crime and thriller fiction, takes place in late January and draws a notable programme of Italian and international writers to the town.