Nepal can be visited the whole year round but depends on the type of outdoor activities and trekking regions you have chosen.
In Spring (March to May): the weather conditions are pleasant with blossoming flora. It is thus the best time to admire the blooming rhododendrons, Nepal’s national flower. In the altitude, there might be some left over snow in some regions. It is the best time to climb Mount Everest and ideal for trekking or other activities.
In Summer or monsoon (June to August): i.e. the rainy season. It is not favorable for trekking. However, some parts of Nepal are suitable for traveling at that time like Mustang, Dolpo, or some places in Manang District which falls behind the rain-shadow. Domestic flight connections might sometimes be on-hold due to the heavy rains. However, the advantage is that you will encounter less people on the trails.
In Autumn (mid-September to November): it is the time of clear sky, trekking, climbing 8000 peaks, and other adventure sports. This period is with spring season Nepal’s high season.
In Winter (December to February): the cold is compensated by the bright sun and brilliant views. It is thus also worth to come for example for a cultural tour to Nepal.