Spring (March, April, May) and autumn (Sept, Oct, Nov) are the best use of the year to visit Nepal. During this time, the weather is pleasant with efficient sunlight and warmth. The sky is obvious which means that you can truly enjoy the remarkable Nepalese landscape complete with the Himalayan vista.
During the monsoon (June, July, Aug) although just about be no problem for trekking, the issue become of less visibility and rain. But, for a keen botanist, monsoon is a blessing as the higher valleys, mountains and meadows blossom with flowers and abundant vegetation. You can trek in winter (Dec, Jan, Feb) as well especially in the hilly regions but as you reach higher elevations it is much more colder with snow-fall. If you don’t necessary enjoy crowds, trekking during the monsoon or winter or choosing more solitary trekking destinations could be your options. Note that due to global warming there has been an increase in the regular climate worldwide and Nepal is no exception. Please be designed to unpredictable weather conditions as well.