Nepal has distinct seasons, and when you go matters a lot — especially if trekking is the plan.
Autumn (October–November) is the classic trekking season and the most popular. The monsoon has cleared, skies are generally sharp and blue, and mountain views are at their best. Temperatures are comfortable: warm in the valleys, cold at altitude but manageable with proper gear. The downside is crowds — the Annapurna Circuit and Everest Base Camp trails see heavy traffic in October, and teahouses book up. Book ahead and start early each day if you want quiet trail time.
Spring (March–May) is the second trekking season. Rhododendrons are in bloom (the national flower), temperatures warm up, and the Annapurna and Everest regions are busy but slightly less crowded than autumn. The higher passes can still carry snow in March and early April. Visibility starts to decline in May as pre-monsoon haze builds.
Monsoon (June–September) is not impossible but requires adjustment. The Terai is hot and humid. The trails are muddy, leeches are out in the lower forests, and mountain views are often obscured. But this is when the hills are intensely green, fewer trekkers are around, and prices drop. Some areas in the "rain shadow" north of the main Himalayan range — Mustang, Manang, Dolpo — are driest during the monsoon, making them the best time to visit these regions.
Winter (December–February) is cold but clear. Kathmandu gets occasional frost, and the high passes are snowbound. Lower elevation treks (Annapurna foothills, Chitwan, Lumbini) are perfectly manageable. The Everest Base Camp trek is doable in winter for fit, well-equipped trekkers, but very cold above 4,000 metres. The Valley's medieval city squares look good in low winter light, and festival crowds are thinner.
For visiting Chitwan for wildlife, October through April is best — animals concentrate around water sources in the dry season and are easier to spot. The park is partly closed during monsoon.
For Lumbini or the Terai generally, October through March avoids the heat and humidity of the pre-monsoon period.