Not on every trail but few on trails, Trekking in Nepal is possible throughout the year. However, the best seasons for trekking are either before or after the monsoon. Mid March to mid June (Spring) and Mid September to Mid December (Autumn) are the best trekking seasons in Nepal. During these times of the year, the weather is generally fine and the skies are clear. Moderate to challenging treks are suggested to undertake only during springs and autumns. Trek like Everest Base Camp, Annapurna Circuit Trek best suit during springs or autumns. We do not recommend treks that wind through trails of sub-tropical forests during monsoon because of the leeches and very slippery trails. During the winters, the high passes on the challenging treks are closed and it is extremely cold so we do not recommend such treks during winters either. During the Monsoon, the treks like Upper Mustang can be done because of its location, which lies at the rain shadow of Mt. Dhaulagiri. Similarly, easy treks like Poonhill Ghorepani, Helambu and Everest Panorama Trek can be done during anytime of the year.
Nepal has a Monsoonal climate with four main seasons. Here we have a General guidance to the conditions at different seasons:
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 snowfall may prevent going above 4,000 - 4,500 metres (13,000 - 15,000 feet). The Jomsom 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 color 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 in Nepal.
If you are planning for the Nepal Holidays, first thing you should think is best time. Although Trekking in Nepal can be undertaken all the time in a year but there are few months which are best for hiking through the rugged path. Autumn and Spring season are the best seasons for travel in Nepal. The peak time for trekking in Nepal is from late September to early December when the air is fresh, calm and cool whereas skies clear providing the best views of the mountains. Spring season is the second best time to trek and climbing to Himalayas.