High season runs from November to March, when temperatures are mild (10–20°C), days are long, and all services are running. December and January are the most crowded — book accommodation and glacier excursions weeks in advance. February and March are slightly quieter and the weather is often more settled.
April to October is Patagonian winter and should-season. Many hotels and tour operators close entirely, and the roads into the park can be icy. If you do visit in shoulder season (April or October), you may have the glacier almost to yourself — but expect cold, unpredictable weather and reduced services.
One weather note: Patagonia is famously windy. Even in summer, afternoon winds can be ferocious. The mornings tend to be calmer, which is part of why glacier excursions often start early.