The atmosphere is fresh and pure due to the monsoon rains, the mountain scenery is superb and the weather is still comfortably warm. December, January and February are also good months for trekking but it will be cold at high altitudes. March, April and May also offer better weather when trekkers will be able to see the varied and beautiful wild flowers, particularly in Nepal's wonderful rhododendron forests. During the monsoon season (June-August) trekking is possible in the rain-shadow areas of north of the Himalaya like Upper Mustang and Upper Dolpo. These regions are out of reach of the rain clouds because of the high mountains and are unaffected by the monsoon.
Irrespective of the weather or the season you are in, Nepal could well be visited throughout the year. Since Nepal is also a launching pad for journeys into Tibet, Bhutan and India (Sikkim & Ladakh), especially during monsoon period, Nepal is a perfect destination for all seasons.