Your chance to fully enjoy the treks in Nepal depends on the crowd and weather. During the tourist seasons, which are the times during which the weather is favorable, the crowd of tourists travelling around this beautiful Himalayan country will be massive and might result into some of you feeling disappointed even though you expect that to be the case. The trails, flights, hotels, lodges, everything will be crowded.
Depending on where you want to travel in Nepal, trekking in Nepal can be undertaken round the year; Nepal has four distinct seasons.
Spring starts from the mid of February and lasts up until mid of June. Temperature during spring varies from mildly warm in the Terai (lowland) belt to relatively cooler at higher altitudes. The sky remains clear most of the time that allows you to enjoy beautiful view of the mountains. Spring is also a flower blooming season. The forests at higher altitudes that you pass by during your trek will be welcoming you with beautifully blossoming rhododendrons.
Heavy Monsoon rains that are brought to Nepal by the ocean currents arising from the Bay of Bengal are characteristic of the summer months of Nepal. The days are hot and mostly wet. Heavy precipitation with thunderstorm and lightning are normality during this time of the year. Monson in Nepal generally lasts from late June to the mid of September.
There are arid rain shadow areas in the Himalayas. Places such as Upper Mustang and Dolpo fall in this region. Summer is the right time if you want to trek in these rain shadow areas. The mountain trails are not at all crowded during this time and the mornings, if not rainy, welcome you by offering clear view of the mountains.
It comes as no surprise that most of the festivals and celebrations of the Nepalese fall in the months of autumn. Autumns are pleasing. The air is clear while the weather is neither so hot nor so cold with very less rainfall. This makes autumn the best time to visit Nepal. However, you have to make do with the crowd of tourists during this time. From the streets of Kathmandu to the rugged trails of the Himalayas, hordes of tourists will be a constant sight during this time of the year. This is the best trekking season in Nepal. The pleasing weather offer the perfect time to walk and enjoy the Mountain View. If you plan to come to Nepal during this season, bookings should be done months in advance. Autumn starts from the mid of September and can last up to the end of December.
If you think that you can bear the freezing cold of the Himalayas, it won't be a problem at all to visit Nepal during winter months- from the mid of December to the mid of February. Generally you are not advised to travel to Nepal during this time. If one wishes to travel during this time, you have to travel with warm down jackets and sleeping bags to fight the cold.