The monsoon rains (mostly at night) bring landslides, and clouds often obscure mountain views. Rain, mud and leeches deter most trekkers, but hefty hotel discounts are common and this is a popular time to travel overland to Tibet.
The second-best time to visit and trek, spring brings warm weather and spectacular rhododendron blooms.
Clear skies and warm days make autumn the peak season. Thousands of people hit the trails in the Everest and Annapurna regions, accommodation in Kathmandu gets booked up and prices peak.