The main trekking season in Patagonia is the 7 month season between October through late April, although trekking year-round is possible.
Trekking between October and November (early Spring) is a great way to beat the crowd and see the fresh Spring growth.
December through March (Summer) always tends to be the best months in Patagonia in regards to weather, and trekking this time of year makes river crossings more tolerable.
March through April (Fall) allows for some cooler and more stable weather patterns. Autumn in Patagonia brings an amazing blast of color, which makes late season trekking a real treat.
May brings cold temperatures and snowfall. Traveling through Patagonia during late or off-season is only recommended for the more seasoned and self-reliant trekkers.
Patagonia in winter is not something to be underestimated. Torres del Paine winter night temperatures are as low as -25C.