Trekking is possible at any time of the year depending on where you are going. The most popular seasons are spring (February-May) and autumn (September-November). Winter is very cold above 4,000 m and high mountain passes may be snowbound, but it is good for trekking at lower altitudes. During the monsoon season (June-August), you can trek in the rain-shadow areas north of the Himalayan like Mustang and Dolpo. These places are out of reach of the rain clouds because of the high mountains and are unaffected by the monsoon.