The best seasons to trek in Nepal are autumn, from mid September until end November and spring, from the beginning of March until the mid May.
Autumn (mid-September to end-November)
This is the most popular time to trek. Generally during autumn, the weather is clear with mild to warm days and cold nights. However, in the higher altitude, the nights drop into freezing temperature. In this season, the mountains views are astonishingly clear.
Approach to winter and the mid winter (end-November through March)
It is also possible to trek during winter, from December until the end of February. Daytime temperatures will be cooler; however, the nights will often be very cold. The days are generally clear but occasional winter storms can bring snow as low as 2500m. Early October through late November is also the busiest period for trekking. But in mid winter (January through March), trekking is more challenging in the high altitudes with semi-regular snowfall followed by more winter storms, which break the long fine periods. The mid-December to mid-February is the coldest time.
As snow gets harder and wind condition remains stabilized in early winter, climbing some of trekking peaks is possible. Climbing Mera peak, Island peak, Chulu, and trekking in Annapurna, Everest, Langtang in early winter have been quite popular over the past few years.
Spring and early summer (mid-March through May)
In the spring, the mornings are usually clear but afternoon cloud build-up brings occasional showers. The days are mix up with warm and rain, which displays wildflowers like rhododendrons. The whole country is lush and an abundant green at this season. This period instigate the second most popular and pleasant trekking season as this is rice-planting time. Late-march into April is especially beautiful. It is also a good time for climbing as the high passes are usually snow free and the mountain views are still clear in April. Up to May, the weather becomes hazy and disturbed with the clouds.
The monsoon (June to mid-September)
From June to September, is the monsoon season. Generally the morning is cloudy and cloud wisps form on random ridges and peaks. Trekking at this time of year is generally difficult and uncomfortable as the weather is hot and it rains almost every day. The trails become muddy and are often leech-infested and the mountains are usually obscured by cloud. During April and May, there is an expectation of thunderstorm, hail shower and strong winds among the fine periods. There are, however, possibilities for summer trekking in the trans-Himalayan regions of mustang, Dolpo and Tibet. These regions lie in a rain-shadow and therefore receive significantly less precipitation than the more southerly areas.