The Himalayas is mountainous so the weather may be unpredictable. We avoid trekking during the rainy season as the experience will not be good, and during winter as it can be extremely cold at altitude.
Generally, trekking is concentrated in the two main seasons - Autumn and Spring.
Autumn(September to mid-November) is the most popular time to trek in Himalaya tourism. In autumn, you are more likely to encounter clear skies and views of the grand peaks. The night temperature might drop below zero.
Spring(mid-March to mid-May) is a pleasant time to trek the Himalayas, especially in Nepal. You will enjoy the spectacle of blossoming rhododendrons and other flowers at this time. The weather is colder than autumn months so please prepare warm stuff for use in teahouses or campgrounds at night but the reward will be quieter trails and spectacular mountain views all around.