The best time to trek is the dry season from October to May. The monsoon period is from June to September. There are two major factors to weigh as you decide when to go to Nepal: Crowds and weather. During the dry season especially in autumn (September-November) and spring (March-May); many people come to Nepal for trekking. This is high tourist season so most of flights and hotels are fully booked and trekking trails can be horrendously busy.
The first two months of the dry season, October and November, offer the best weather for trekking. The air, freshly washed by the monsoon rains, is crystal clear, the mountain scenery is superb and the weather is still comfortably warm.
The December,Janauary and February months are still good for trekking but cold can be bit effected in high altitudes. Getting up to Everest Base Camp can be real endurance test and High Passes, especially Thorung-La on the around Annapurana trek, and Lourebina Pass on the Gosaikunda trek are usually closed December to February months.
In spring (March-May) the weather has become dry for a long time and dust is starting to hang in the air, affecting the middle hills especially around Pokhara but the high country is usually clear. You will be welcomed superb views of wildflowers rhododendrons on trekking trail in this time.
The monsoon is a good time to visit Kathmandu and Pokhara, 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.