Trekking is possible at anytime of the year depending on where you are going. The most popular seasons are spring (February-May) and autumn (September-December). Winter is very cold above 4000m 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, upper Manang and Dolpo. These places are out of reach of the rain clouds because of the high mountains and are unaffected by the monsoon. During the monsoon in Nepal it is recommended to travel in Tibet (May to September)
Monsoon season (June – August) is considered to be the most unattractive time for tourist activities due to the heavy rains. Many people decide not to visit Nepal during this period of time, especially after reading some disappointing information regarding the monsoon definition. Even though the amounts of rains are considerably higher compared to the peak season, there are still plenty of activities and opportunities for adventure. Many first time trekkers who trek during the monsoon are still very satisfied with their first experience. Also what makes the trek more unique and personal is the extremely small amount of tourist on the trek compared to peak season. You can avoid monsoon by trekking in the rain-shadow areas north of the Himalayas (Mustang, Upper Manang and Dolpo). Rain clouds simply reach these areas because of the high mountains, therefore they are unaffected by the monsoon.