Trekking is possible through out the year. However the best seasons for trekking are either side of the monsoon season. Mid March to mid June and Mid September to Mid December are the best. During this time the weather is generally fine and the skies clear. Beside this season, it is possible to trek in other season,however we can expect lots of rain (and leeches) during the monsoon and severe cold and closed passes during the winter months. Rain shadows areas like Annapurna Circuit, Upper Mustang are recommended trek during monsoon season & Lower altitude treks like Ghorepani, Langtang Valley, Namche, Helambu are recomended trek during winter season.
Nepal has a Monsoonal climate with four main seasons.Traditionally a year was categorized into six distinct climate periods: Basanta (spring), Grishma (early summer), Barkha (summer monsoon), Sharad (early autumn), Hemanta (late autumn) and Shishir (winter).
Heavy monsoonal rains from June to September – the rains are generally lighter high in the Himalayas than in Kathmandu, though the mountain peaks are often not visible due to clouds. In the Kathmandu Valley & Pokhara – monsoon rains typically consist of an hour or two of rain every two or three days. The rains clean the air, streets, & cool the air. If you come, bring an umbrella, expect lower lodging prices & fewer tourists.
Clear and cool weather from October to December – after the monsoon, there is little dust in the air so this is the best season to visit the hilly and mountainous regions.
Cold from January to March, with the temperature in Kathmandu often dropping as low as 0°C (32°F) at night, with extreme cold at high elevations. It is possible to trek in places like the Everest region during the winter, but it is extremely cold and snow fall may prevent going above 4,000 – 4,500 metres (13,000 – 15,000 feet). The Jomosom trek is a reasonable alternative, staying below 3,000 metres (10,000 feet) with expected minimum temperatures about -10°C (14°F) (and much better chances of avoiding heavy snow.)
Dry and warm weather from April to June – there is an abundance of blooming flowers in the Himalayas at this time, with rhododendrons, in particular, adding a splash of colour to the landscape. Terai temperatures may reach or exceed 40°C (104°F) while Kathmandu temperatures are about 30°C (86°F). This is the best time to undertake mountain expeditions. Trekking such as Everest basecamp trekking, Annapurna basecamp trekking, Manaslu region trekking, Mustang Region trekking and many more trekking can be seasoned.