The best time to trek to Upper Mustang is in July, August and September to see the vegetation of Himalayan arid plateau. Second best time to trek there is March to Mid June. Mustang is under the rain shadow, so when it is monsoon (Mid June to August) in Nepal; trekking to Mustang is good time. And it is possible to trek Mustang between all around the year except winter (Dec to February) as your trek may be affected by snowfall.