Generally, in Nepal spring season (March – May) and autumn season (September - November) is considered as the best season for trekking as well as tour, but leisure tour and city sightseeing tour can be done in February, June, April, May and December as well.
The Nepal trekking season are classified as follows:
Autumn season (September – November)
In Nepal Autumn season is considered as the excellent trekking season. September, October and November are the months which fall in this season. In this season weather are very good which offers awesome views of hills and mountains during treks. In this season temperature is moderate which makes easy to travelers for leisure tour, hiking, trekking and climbing in mountains. During this season, mostly sky is clear which provides outstanding views but occasionally short storms dump cloud at high altitude.
Winter season (December – February)
December, January and February are the months which fall in this season. In this season usually snowfall occurs at higher elevations in Himalayan region. Though it gets colder in night & morning time, often it offers the clear weather/blue sky and relatively less hikers on the route. Trek like Annapurna Base camp, Everest Base Camp, Langtang valley trek, Ghorepani Poonhill can be done but high pass trekking is not suitable in this season.
Spring season (March – May)
March, April and May are the months which fall in this season. This season is also considered as the perfect season to do the trekking, climbing and expedition. During this season offers giant rhododendrons and different varieties of flora and fauna above 3000 meters elevations which make to feel trekking more interesting and exciting. In lower elevations temperature is warm and moderate at higher elevation which makes trekkers more comfortable for trekking.
Summer season (June – August)
June, July and August are the months which fall in this season. This season is known for rainy season in Nepal. For trekking and tour this season is ideal time. However, some trekking regions connecting to Tibet like Upper Mustang, Dolpo and Narphu valley are best trekking route to do trekking in summer season.