There are two major factors to consider as you decide when to go to Nepal: crowds and weather. As a general rule, the better the weather, the more people.
Nepal has four distinct seasons:
The middle hills, especially Pokhara, are full of dust and haze in April and May, but the high country is usually clear. The hottest month is May, just before the monsoon. Trekking tapers off in the heat of May except at high elevations.
Summer is monsoon season and is considered the “off-season” for trekking. Trails are not crowded but are slippery, wet, infested with leeches, and hot. People rarely are lucky enough to get a glimpse of the Himalayas through the clouds. Many who are willing to
put up with the rain still trek at this time (typically rains only in the afternoons/nights). Prices are usually discounted during this season.
Lukla, Jumla and Jomsom are above the leech line so this is also a good choice for treks during the monsoon. Mustang and Simikot are partially in the Himalayan rain shadow, so conditions are less wet.
Autumn is the most popular travel time as the temperature is cooler, the monsoon has passed, and the mountainsides are vibrant green from the recent rains.
Nights are cold in the mountains, but the bright sun makes for pleasant day temperatures – in the high 20’s Celsius, falling to 5 Celsius at night (1,000 – 3,500 meters). At higher altitudes temperatures range from about 20- (-10) Celsius.
Winter is about 10 degrees colder than Autumn. Early December usually has a lull, and is a good time to trek. High passes, (Thorung La Pass on the Annapurna circuit and Laurabina Pass on the Gosainkund trek) are usually closed from late November to March due to snowy conditions. February is less cold as the spring trekking season approaches.
There are always exceptions to this weather pattern, so be prepared for extremes!