There are two major factors to weigh as you decide when to go to Nepal: crowds and weather. As a general rule, the better the weather, the more people come to Nepal to go trekking. During the high tourist season in October and November, flights and hotels are fully booked and hotels and trails in the hills can be horrendously busy.
During autumn the nights are cold in the mountains, but the bright sun makes for pleasant day temperatures - in the high 20s° C, falling to 5° C at night, between 1000 metres and 3500 metres. At higher altitudes temperatures range from about 20° C down to -10° C. Mornings are usually clear with clouds building up during the afternoon, disappearing at night to reveal spectacular starry skies. During winter it is about 10 degrees colder.
Early December usually has a lull, but this is also a good trekking season. The Christmas period is cold, but this is the holiday season in Japan and Australia and these nationalities dominate flights and hotels. High passes, especially Thorung La on the Around Annapurna trek and Laurabina Pass on the Gosainkund trek are usually closed from late November to March. February is still cold, though less so as the spring trekking season of March and April approaches. The Middle Hills, especially around Pokhara, are full of dust and haze in April and May, but the high country is usually clear. Trekking tapers off in the heat of May except at high elevations.
The monsoon is a good time to visit Kathmandu, but there are few trekkers among those who come. A monsoon trek is possible if you are willing to put up with the rain, leeches, slippery trails and lousy mountain views. Flights operate throughout the monsoon to Lukla, Jumla and Jomsom, so it is possible to fly in and trek above the leech line.
Many of the new treks to recently opened restricted areas are good summer treks. Mustang and Simikot are partially in the Himalayan rain shadow, so trekking conditions are good throughout the monsoon season. Most of the restricted area treks are impossible during the winter season.