You can visit Nepal year-round. Different activities are better at different times of the year. The best time to visit Nepal for trekking is Autumn (mid-September to end-November) and spring (mid-March to end of May). During autumn, weather is generally clear with mild to warm days and cold nights. The skies are generally clear and provide amazing mountain views. During spring, the morning is usually very clear but the afternoon cloud brings rain showers. Clear skies and good views are still possible this time of year. Furthermore, during the spring the wonderful Rhododendron flowers are in full bloom and the jungle forest is lush.
However, in the winter (end-November to mid-March) the Himalayan area becomes freezing cold and trekking is more challenging. But in some part of the low hills, you can still trek comfortably. Lastly, the summer marks the monsoon season. During the monsoon season (June to mid-September), the morning is cloudy. Trekking is very uncomfortable because the trial become muddy and leech-infested. Manang and Mustang treks can be done in this season. The summer is a good time for safaris and city tours.