In term of best time to visit Nepal depend what type of trip you are doing in Nepal. Nepal is broadly temperate with four main seasons. Its climate varies as per topography and altitude. Nepal is having varies altitude. The low range of Tarai region is maximum altitude of 305 miters, hilly region in between Terai and Himalayan region is from 500 miters till 3000 miter altitude. The maximum altitude at mountain region is up to 8848 miter Mount Everest.
Autumn is the main season for Trekking in high mountain region. (September to November)
Autumn season is best season for trekking in Nepal. Weather is clear, dry and neither cold nor hot. Dust and pollution washed away by the monsoon rains. The mountains are most visible. This is considering as best season in Nepal for trekking in high mountain region. It is better to book your trip an advance, since hotels and flights are booked from previous. Major hindu festival Dashain and Tihar is also during this season. Beside trekking, any kind of tours in Nepal is suitable during these months.
Spring is second high season for trekking and expedition in Nepal (March to May)
Spring season is mildly warm weather and longer days. It is also best time for trekking in high mountains and viewing wildlife. The major expedition including Mount Everest trip runs in these months. Flowers blooming time makes colorful hilly and low level of mountain region. The different colored Rhododendron flowers bloom at mountain region makes fascinating together with clear Mountains peaks.
Winter is low touristic season (December to February)
Weather in winter in Nepal is mostly clear and stable. Days are sunny but morning can be chilly. You still can face mountain cold at this time. Trekking is lower altitude runs well even though some of the high mountain trekking like, Everest Base Camp Trekking, Annapurna Base Camp Trekking, Gokyo valley trekking, Langtang valley Trekking without crossing any icy passes at high altitude. You could see very gorgeous view of mountain peaks with its fully ice. Some of the peak climbing activities in this season still goes.
Monsoon season low touristic season (June to August)
Monsoon season in Nepal is almost raining every day. Temperature is hotter. Mountains are mostly cover by fog after afternoon time. Morning time you normally will get very fresh view of peaks. Greenery hills, fresh air, movements of different butterfly make your Nepal trip more fascinating. There are some rain shadow trekking are in Nepal which is more suitable to trek at monsoon season. Upper Mustang Trekking is one of those.