Nepal has five seasons (winter, spring, summer/monsoon, and autumn), and weather/climate varies from one region to another as per the season.
(End of November to end of Feburary) Winter is, of course, the coldest time of the year. The heavy snowfall and snowstorms in the mountains and unclear visibility make trekking unpleasant. Kathmandu and the Terai region do not receive snowfall. However, morning and night get quite cold. The temperature falls below 0°C in the mountains.
Mid-September to end of November welcomes the autumn season. The weather usually is a little humid, with warm and sunny days. It is the best time of the year to trek in the Himalayas. With stable weather and a moderate climate, the trek becomes fantastic. The views are clear, and the trails get decorated with a range of colorful flowers. Sometimes the temperature falls to 0°C.
Spring- March to May is the second-best time of the year to trek in Nepal. Moreover, most of the 8,000 expeditions occur in this season and extend up to May. The trail blooms with beautiful flora, and the climate begins to warm up after an extreme cold.
Summer/monsoon (June to September) is not a good time to travel in most parts of Nepal, especially in the mountains. The heavy rainfall not only ruins the views but also makes the trail slippery and prone to landslides.There are two regions where you can trek in the monsoon- Upper Mustang and Dolpo. These two regions fall under the rain shadow area, and because of that, both regions do not receive any rainfall in the monsoon.