Spring season (March - May):
The second most popular time of the year for trekking and climbing in Nepal is from late February through April. Spring is a colorful season, different varieties of wild flowers specially the rhododendrons. The temperature is mildly warm at lower elevations and the temperature is moderate above 4000m and the mountain views are excellent. Holi and Shivaratri festivals are also celebrated during this season.
Summer/Monsoon season (June - August) :
Travel in Nepal during summer months are wet and warm. It is not recommended for trekking and climbing except in the rain shadow regions like Langtang, Dolopa and Upper Mustang. In the higher valleys, meadows blossom with flowers and lush vegetation. It is fine to travel to Tibet, Sikkim, Ladakh (India) and Bhutan during monsoon.
Autumn season (September - November):
Autumn is one of the best seasons to travel to Nepal. It starts from early September to early December, generally during autumn; the weather is clear with mild to warm days and cold nights. In this season, the mountain views are astonishingly clear. Also, during autumn there are all kinds of tourist activities. It is the best season for trekking and climbing, flowers blooming and Nepal is lush with an abundant green.
The main festivals in Nepal, Dashain and Tihar (Deepawali) are celebrated in autumn. It is the best time to experience your travel in Nepal with cultural, history, nature and wildlife.
Winter season (December - February:
Winter season in Nepal starts from late December and continue to till March. The cold season is best for sightseeing and cultural tour in Nepal. Jungle safari, bird watching, city sightseeing tours, and mountain biking are the popular activities in winter. Due to the frozen of snow, the mountain rivers are low volume water, so rafting during this season is popular one.
Trekking in the lowland of Nepal are possible but in the mountain heavy snowfall might be the problem for trekking.