When to go to Nepal:-
Much of Nepal enjoys a temperate climate throughout the year with four distinct seasons. Many consider autumn (September, October and November) the best time to visit it (especially those who want to go trekking) as following the monsoons that characterize the summer and take away pollution and dust, the visibility of the mountains is optimal as well. Like the temperatures that at low altitude are not too high and at altitude they are not too low. In addition, two of the most important festivals in the country - Dasain and Tihaarl - take place during this period.
As you may have understood, autumn is by far the most expensive and crowded period (especially the month of October) with accommodations that are constantly sold out at high altitude along the main treks.
In any case, Nepal can very well be visited throughout the year, as long as the activities and destinations involved in the trip adapt to the period, here is an overview of the other seasons.
Winter (December, January and February): It offers a generally sunny and stable climate. It is a great time to visit all the Terai, that is the region south of the country at the foot of the Himalayan giants that includes Kathmandu, where the rather low average altitude means that snowfalls are absent and although the mornings can be decidedly cool, the rest of the day offers pleasant temperatures.
In this period, trekking at low altitude is possible and with the right equipment, except for heavy snowfalls, it is technically possible to climb to altitude and complete some of the most popular treks such as the one to the Everest base camp and the Annapurna circuit but of certainly not the ideal time to do it.
Spring (March, April and May): This is the second most popular season to visit the country. The temperatures and visibility for trekking are good but humidity often builds up in the afternoon and the peaks are covered by clouds.
In any case, the advantage of having less crowded paths combined with the flowering of numerous plants and flowers (rhododendrons above all) make spring a great place to visit the country, especially for those who want to go trekking and avoid the very high autumn season.
Summer (June, July and August): This in Nepal is the period of monsoons that typically arrive during the first weeks of June and can also last until September. It is certainly an interesting time to visit the country with nature literally coming to life, the air is clean and butterflies are everywhere. Trekking is however largely discouraged, except for some areas known as "rain shadow". Here the precipitation is considerably lower than in the rest of the country, one above all is the northern part of the Annapurna circuit (Manang, Upper Pisang, Muktinath) where rainfall is decidedly lower, in any case especially at low altitude the risk of roads blocked due to landslides and the presence of leeches are definitely things to consider.