Seasons without rainfall or snowfall are the best time to trek in Nepal .During monsoon or winter the trails in the mountains are blocked. For high altitude treks October to early December and March to May are the best time. At lower attitudes like Jomsom , treks can be organized even in December. For regions further lower like Poonhill or Panchase treks can be organized all through the winter. However in rain shadow zones like Mustang and Dolpo treks can be organized even during summer .These areas experience very little rainfall in summer and it is warmer that time of the year. The barren, alpine meadows turn into green carpet of grasses and bloom with verities of alpine flowers.
Sep-Nov October and November are really Nepal’s highest months – offering excellent weather and tantalizing mountain views.
Dec-Feb This season is noted for occasional snowfall only at higher elevations. Mountain passes may be snowbound, hence it is ideal for trekking at lower elevations, generally below 3000 meters.
Mar-May Different varieties of wild flowers, specially the rhododendrons, make the hillside above 5000 meters a haunting paradise during this season. It is mildly warm at lower elevations and at higher elevation over 4000 meters the mountain views are excellent and temperature is quite moderate.
Jun-Aug Summer months really continue up to mid-September making travel wet and warm. These times are blessed for the keen botanist as the higher valleys and meadows blossom with flowers and lush vegetation. Insect repellent is a nice addition to your What to Bring list. During this season, you can trek in the rain-shadow (places are out of reach of the rain clouds because of the high mountains) areas north of the Himalaya like Mustang and Dolpo.