You can trek Nepal throughout the year depending on the region. Nepal has tremendous variation in climates due to the vast differences in altitude. The low land plain are sub tropical (600-1200m), the midland hills are warm temperate (1200-2100m) and cool temperate( 2100-3300m) and the high mountains are Alpine (3300-5000m) and Tundra (5000-8848m). Mid of September to mid of December is considered the best Trekking season in Nepal.
Autumn Season (September-November)
Autumn is considered the best Trekking season in Nepal. You can experience better visibility of tantalizing mountains, green hills and excellent weather. Weather is generally clear with mild to warm days and cold nights. The possibilities of natural disasters diminish, making it a safe season to trek. The Nepalese festivals Dashain and Tihar are celebrated during this time.
Winter Season (December- February)
In winter, the Himalayan region freezes over and the snow falls. Trekking can be challenging in the higher altitudes because the trail can be covered with snow. In the winter you should trek at lower altitude below 3000m. The temperature of Himalayan region will be up to -18 degree Celsius, hilly regions 0 to 5 degree Celsius whereas in plain regions 10 to 15 degree Celsius.
Spring Season (March-May)
The blooming flowers, lush greenery, and sparkling mountains make for an excellent background to spring trekking. Spring feels like a delight with singing birds, beautiful views, and abundant wildflowers. Weather will be mild at lower elevation but cold at higher elevation. In spring, mornings are usually crystal clear. However, in the afternoon clouds bring rain showers.
Summer Season (June-August)
In summer, mornings are cloudy with rain in the afternoons and evenings. Trekking is very uncomfortable because the trial becomes muddy, wet, and leech-infested. You risk injury in the wet terrain. Furthermore, there is higher chance of natural disasters; mainly landslides in the hilly regions. If you trek, you must not forget to carry insect repellants. The temperature of Himalayan regions will be 16 to 20 degree Celsius, hilly regions 20 to 30 degree Celsius whereas the plain regions 34 to 39 degree Celsius. Although this is not a good season to trek, the summer is best for botanists and researchers in forest and higher altitudes. However, places like Manang, Mustang and Dolpo are located in a rain shadow. Summer is the best time for these regions. Temperatures are mild and the weather is not so fierce.