Trekking in Himalayas of Nepal can be done through the year, but to get most out form the trekking destination- the best time of the year is September-December (autumn) & March-May (spring).

The reason behind these two seasons is-The weather.
During these months, the weather is clear, sunny & pleasant for you to enjoy the Himalayan vistas with least effort. You don’t need rain gears on your backpack since rainfall is not expected on these seasons. The month of May is most rewarding because the rhododendron forest will blossom making it so delightful to watch.