Nepal is a pleasant country to visit throughout the year. That being said, there are benefits and drawbacks to visiting during each season. Typically, the best weather happens before and after the monsoon, when the skies are clear and the temperature is moderate to warm.
Autumn (September until mid-December)
Enjoy clear skies and glorious mountain views during Autumn. Temperatures in the mountain region are warm during the day, and cool/cold at night.
Before the monsoon, temperatures start to warm up and the colorful alpine flowers are in bloom. Rhododendrons and magnolias liven up the magnificent views. The temperature is warm during the day and cool in the night. Also, the skies are still open for awesome mountain views.
The rain only stops for brief moments during the monsoon season. The mountain views are not ideal, and leeches start to occupy the muddy trails. This is the season that most people typically avoid, though because of this, there are less tourists on the trails. The air is hot and humid during the day and moderate/warm at night.
During winter, the weather is cool during the day and cold at night. Skies are crystal clear, and you can expect some snowfall at around 2500 meters and above. If you can handle the cold, the winter is an amazing season to trek in the Himalayas.