Nepal has 4 seasons these are:
Spring: March to May. It is warm and quite dusty with a few rain showers. March and April is a good time for trekking too. The sky is a bit dustier, but you normally get good views in the mountains. With Rhododendron, Nepal’s national flower, being in full bloom, walking through the forests is particularly nice.
Summer: June to August. It is the monsoon season, and it normally rains a lot during these months. During the monsoon from June to August, trekking is not very popular due to the heavy and frequent rain showers. However, the vegetation becomes very green and lush, which makes for beautiful sceneries. In Upper Mustang, Humla and Dolpa, summer is an excellent time for trekking! These areas lie in the rain shadow of the main Himalayan range and are therefore not affected by the monsoon. Getting there might be a challenge though due to unpredictable weather conditions in the Lowlands, which generally affect flight schedules.
Autumn: September to November. It is cooler with clear skies; this is the most popular trekking season. October and November are the most popular trekking months in Nepal. During this time, chances are high for clear skies and great mountain views. The nights are usually cold in the mountains, but the bright sun makes for pleasant day temperatures (above 20s° C, falling to 5° C at night between 1,000 m and 3,500 m). At higher altitudes temperatures range from about 15 ° C- 20° C during day time to -10° C at night. Mornings are usually clear with clouds building up during the afternoon.
Winter: December to February. It is cold at night and it can be foggy in the early morning. The afternoons are usually clear and pleasant at lower altitudes, though in the mountains it is quite cold and lot snow. December, January and February are good for trekking as long as you stay below 3,000 m. Above 3,000 meters it gets very cold, and trails are often covered with snow. It is usually about 10 degrees colder than in autumn. High passes (above 4,000 m) are generally closed from Mid-November to Mid-March.