Trekking in Nepal can be undertaken throughout the year depending on the region. The year is divided into 4 different seasons in Nepal and each season has its distinct attraction to offer for trekking.

Autumn (Sept-Nov) :
This is the time when you can expect better visibility, the air is fresh, and the weather is perfect and that the Nepalese most important festivals (Dashain and Tihar) are marked during this season The best season offering excellent weather and tantalizing mountain views.
Winter (Dec-Feb) :
This season is noted for occasional snowfall and the air can be quite cold at higher elevations. Hence it is ideal for trekking at lower elevations, generally below 3000 meters and offers tantalizing mountain views.
Spring (March-May) :
Trekking in spring is particularly lovely as the mountain slopes are covered with beautiful and colorful flowers and still have plenty of high snow to enhance your photos. The temperature is quite moderate and the mountain views are excellent.
Summer (June-Aug) :
This is the favorite season to trek in the rain shadow areas like Upper Mustang, Dolpo and Kailash. This season is also recommended for forest researchers and botanist.