Mid September through April. Camping La Junta also opens and closes around these dates. Outside the season tends to be humid, cold and too rainy, main river crossings closed, and generally not recommended to go.
Best for hiking:
All season. The valley's beauties are great to see and hike all season. Spring's (November through December) waters are high making waterfalls gushing and gorgeous. Many of snow filled peaks, though, can't still be accessed. By March days begin to shorten, the February crowds disipate and cooler hiking temperatures appear. In April, the forests mushrooms flourish, and the snowy peaks have mostly melted thus enabling hiking access.
Best for climbing:
December through March. Depending on the amount of winter's snow fall, melting snow can streak down walls and their cracks until late November to mid December. North facing walls, like Trinidad, dry the quickest. December can be the best climbing month if the conditions are right. The days are at their longest (15 hours of light) allowing more time to complete long routes in one push. Tabano flies have not yet come out. December tends, however, to be rainier than January or February. January and especially February is generally the dryest yet also the busiest. March can also be great, but the days become noticably shorter. April days are even shorter, cooler and generally rainier.
Best for peace & quiet,
Best for saving $:
Avoid January & February if you're planning and basing your trip on a budget or looking for escaping the beaten tourist path. These months are the busiest thus the high-season prices. Accommodations and public transportation can be full throughout the South due to swarms of vacationing Chileans. Cochamó's peaceful nature can at times be lost among-st the multitudes.
Months to avoid:
May through October. Although there are the occasional sunny days, much of the days sun is lost behind the northern mountains. The days are very short. The Refugio Cochamó and Camping La Junta, furthermore, are closed. Many river crossing are removed making moving around the valley difficult or impassable. Plus it's rainier, muddy, more humid and cold.