Nepal basically has four seasons: Spring (March-May), Summer (June-August), Autumn (September-November) and Winter (December-February).
However, owing to its varied geography, weather conditions of Nepal vary from one place to another. The higher you give up north, it tends to be cooler whereas the deeper the south you go the hotter it is comparatively.
In the hilly and Himalayan regions, summers are cool and balmy and winters are severe, while in tropical plains of the Terai in the south, summers are tropical and winters are mild. The temperatures in the valleys of Kathmandu and Pokhara tend to be pleasant with average summer and winter temperatures.
The temperature ranges from below zero to 25 degrees in the Hills and Himalayas where as it can reach up to 35 degrees in flat lands of Terai.
The monsoon rain fall occurs during the summer. The average annual rainfall is 1,600 mm, but it varies by eco-climatic zones. Travelling in Nepal is possible throughout the year.
There are multitudes of trekking areas you can visit in Nepal throughout the year. Nevertheless, the best time to do trekking are during spring and autumn. These are also the seasons when many of the biggest festivals of Nepal are observed.