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 bring 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.

The best time to visit Nepal depends on what you want to do and see while you're there. Generally speaking, the best time to visit Nepal is during the autumn months of September to November or the spring months of March to May, when the weather is mild and dry. Here's a breakdown of what to expect during each season:

Autumn (September to November):
This is the peak tourist season in Nepal, as the weather is clear and dry, and the skies are usually free of clouds, providing excellent views of the mountains. This is also the time when major festivals like Dashain and Tihar take place, offering visitors a chance to experience Nepali culture and traditions.

Winter (December to February):
Winter in Nepal can be cold, especially in the higher elevations, but it's still a good time to visit if you're interested in trekking. The skies are usually clear, and the trails are less crowded than in the peak season.

Spring (March to May):
This is another popular time to visit Nepal, as the weather is mild and dry, and the forests and hillsides are in full bloom with rhododendrons and other wildflowers. This is also a good time to trek in the mountains, as the snow begins to melt, and the trails are not as muddy as during the monsoon season.

Summer/Monsoon (June to August):
This is the least popular time to visit Nepal, as the weather is hot and humid, and the monsoon rains can make trekking and other outdoor activities difficult. However, this is a good time to visit if you're interested in seeing Nepal's lush green landscapes, and the prices for accommodation and flights are usually lower than in the peak season.

Ultimately, the best time to visit Nepal depends on your interests and what you want to do while you're there. It's always a good idea to check the weather and climate conditions before planning your trip.

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.