Best Trekking Season and Weather condition in Nepal

Trekking in Nepal can be organized throughout year and at any season; however, there are four-different seasons in Nepal-- Winter, spring, monsoon, and autumn.
The winter is cold and snows in the Himalaya region but trekkers are able to go for hiking and trekking during this season. The trekkers especially need good and warm clothes and sleeping bag during this season. Tourists can see different things at different season.
The spring season is cold but temperature goes warm day by day. So spring season is best for hiking, trekking in the mountain region. March, April and May fall in this season. This is the best season for blooming flowers. Around March and start of April is cold but end of April to May is start warm.
The monsoon is the rainy season. Travelers go for hiking and trekking less frequently during this season. Of course people can get different adventures at every different season. June, July, and August fall in this season. August is best seasons for trekking in Dolpo and also a few other different part of the country.
Autumn is considered to be the season especially for trekking as there is not so warm and not so cold. Majority of trekkers visit Nepal during this season. September, October, November and December fall in this season. The warm weather begins from the beginning of September and continues to October, November and at the end of December there starts cold season.
Further explaining; October and November: Clear sky and generally good weather. Warm in the day, cool at night. Above 3,000 m, nights may be freezing. Major trails are crowded. Get to the day’s destination early to ensure a place.
December and January: The coldest months, with nights below 0 degree Celsius above 3,000 m. Beautiful days and clear views. Occasionally, at higher altitudes it snows.  
February and March: A transitional period. February is cold. March is warm at lower attitudes. There are chances of rainstorms, high winds, thunder, lightening, and hail. Haze and dust are pushed up from India, dimming mountain views from lowland, but the rhododendron begins to blossom.
April and May: Walking below 1000 m means dripping sweat. You better drink plenty of water. The mountain views are hidden behind the haze. Occasional rain clears the sky for a day or two. Rhododendron blossoms turn entire hillsides red. It is the ideal time for naturalists. Up high, it is still cold.
June, July, August, and September to Mid-June: The monsoon starts and brings relief from the heat. Clouds hide the mountains, but there are occasional magnificent views. Trekking is difficult, with slushy trails often turning into streams or being swept by landslides, leeches are out in full force. Rain-swollen streams may be impassable for hours or days. Rain shadow regions such as the Muktinath to Jomsom, Khumbu and Upper Langtang areas can be trekked during these months. Preparation for trekking
Geographical Variation and Trekking Preferences

Nepal is mainly divided into three main geographical regions:
Himalaya Regions: This region accounts for about 15 percent of the total land areas. The altitude of this region ranges between 4,800 m. to 8,848 meter. It includes eight of the 14 highest peaks in the world which exceeds an altitude of 8,000 m. Those highest peaks in Nepal are, Mt. Everest (8,848 m), Mt. Kangchenjunga (8,586 m), Lhotse (8,516 m), Mt. Makalu (8,463 m), Mt. Cho Oyo (8,201 m), Mt. Dhaulagiri (8,167 m), Manaslu (8,163 m), and Mt. Annapurna (8,091 m).
Mountain Region: This accounts for about 68 percent of the total land area. It is formed by the Mahabharat range that soars up to 4,800 m and the lower Churia range.

Terai Region: The low-land Terai occupies about 17 percent of the total and area of the country. It has a width of about 26 to 32 kilometers and an altitude maximum of 305 meters. Kechanakawal, the lowest point of the country situated in Jhapa district (In the eastern part of the country) lies in this region, it has an altitude of 70 meters.