Most recommended seasons for trekking are autumn (Sept, Oct, Nov) and spring (March, April, May). In these seasons you will be rewarded by good weather, sunny and warm with clear sky and outstanding views. During monsoon (June, July, Aug) although there will be no problem for trekking, the issue could be of less visibility and rain. But, for a keen botanist, monsoon is blessing as the higher valleys, mountains and meadows blossom with flowers and abundant vegetation. You can trek in winter (Dec, Jan, and Feb) also; only the issue is cold weather with snow-fall at higher elevations.
The trekking routes are crowded during spring and autumn but during monsoon and winter the routes are not packed and you could enjoy rather best of nature.
However, because of effects of global warming the climate is changing world wide and there could always be possibility of exception in these weather patterns. In best seasons also there could be hoax of bad weather and sometimes even in adverse months; weather tends to be excellent for trekkers.