If you are planning to come to Nepal, obviously the weather is one of the important factors to consider. The best time to travel in Nepal is from March to April (spring) and October to November (autumn). Some specific activities can be done in monsoon and winter too. Each season has something special and offers a unique experience, either it is in trekking or tour.
Autumn (October to November)
It is the most popular time of the year to trek or tour in Nepal. The trails see trekkers from all around the world. The weather is pretty warm, and the clear sky makes the trek mesmerizing. Similarly, the views are at their best. The temperature may drop to zero at a higher altitude at night.
Winter (December to February)
Winter is not an ideal time to go on a long adventurous trek because of the freezing cold. There are few treks like Everest Panorama Trek, Everest Heli Trek, Annapurna Base Camp Heli Tour, etc that you can do in winter. The goal is to maintain low altitude while trekking in winter. Do know that you may have to face heavy snowfall, unstable weather, and flight cancellations. The views of mountains during winter are the best.
Spring (March to April)
Spring is a clear season. The climate begins to warm up, and the flowers decorate the trails. Trekking in spring is beautiful. It is the second most popular trekking season in Nepal. The mornings and nights remain relatively cold.
Summer/Monsoon (May to September)
Monsoon is not a good time to trek. In early May, you can trek as the chances of rainfall is pretty low. Likewise, it is also peak time for mountaineering. While other months are not recommended for trekking. You may have to face heavy rainfall, landslides, and lots of bugs on the trail.
You can go on Upper Mustang Trek or Dolpo Trek in the monsoon because they fall under the trans-Himalayan region, which means little to no rain in the monsoon season.