Short answer
Mid-September to late November is the best trekking season in Nepal, with clear post-monsoon air and stable weather. March to mid-May is the second season, warmer and greener but hazier. Monsoon, June to August, is a poor idea on most trails — except Upper Mustang and Dolpo, which sit in the rain shadow.
Autumn is the reason October is the busiest month on every major trail. The monsoon has washed the haze out of the air, so mountain visibility is at its annual best, and daytime temperatures are comfortable at every altitude. The cost is company: Namche and the Annapurna lodges fill, and beds need booking well ahead.
Spring trades views for flowers and warmth. Rhododendron forests on the Annapurna and Everest approaches are in bloom from March, nights are milder, and it is the main season on the big climbing peaks. Haze builds through April and May, so the long views are softer than in autumn.
Winter is clear, quiet and cold. Trails below about 4,000 m are perfectly walkable in December and January; higher lodges close and passes can be snowed shut. Monsoon is genuinely wet, with leeches, cloud and unreliable flights — but Upper Mustang and Dolpo lie north of the main range and are at their best precisely then.
| Season | Months | What you get | What you give up |
|---|---|---|---|
| Autumn | Mid-Sep – Nov | Clearest views of the year, stable weather | Crowded trails, book lodges ahead |
| Spring | Mar – mid-May | Rhododendron, warmer nights, climbing season | Haze softens the long views |
| Winter | Dec – Feb | Empty trails, sharp light, lower prices | Cold, unheated lodges, some passes closed |
| Monsoon | Jun – Aug | Green valleys, no crowds, cheap | Rain, leeches, cancelled flights |
| Rain shadow | Jun – Aug | Upper Mustang and Dolpo at their best | Restricted permits and higher cost |