Nepal is an all season destination for trekking. As Nepal has so many wonderful trekking peaks, some are better on autumns, some good for spring and some for monsoon. This no-specific-time rule applies very well for trekking in Nepal. The journeys can be taken any time of the year, depending on where you want to be going.
Feb and March are the best time for trekking any peaks as the days are sunny. At night time it can get cold and some higher places still sees snow. Actually at this time of the year, all the peaks are covered with snow. It is at this time when you can experience the wild beautiful flowers on the jungles and crystal clear river from the melting of snow.
April-May is not a popular season for trekking, which makes it a whole lot better because it is less crowded. Spring brings the occasional pre-monsoon rain which makes it a little difficult for trekkers, but yet it gives them a chance to relax for a moment and get to know the original inhabitants. From April the weather gets hotter and the days can be bright and sunny, It is another great time for trekking. You can take a good sun bath on the day and feel the cold breeze of snow at evening and night.
Monsoon is another good season for low altitude areas. Monsoon (is very pleasant if the heavy downpour is to be ignored at times when one is enjoying a walk in the streets of Nepal. However, it a very good time to trek the rain shadow area in Manang and Mustang.
Autumn is definitely the perfect time for trekking. The skies are clear and the weather is absolutely fine. Being the busiest time of the year for trekkers, it is also the most crowded time of all. Kanchenjunga, Manaslu, Dhaulagiri are the tracks to be followed during this time of the year. The morning are a warm welcome for a great start for day, which makes Autumn the star season for trekkers willing to heat the mountains in Nepal.