October through November is probably the best time to visit Nepal. The weather is balmy, the air is clean, and the visibility is at its best. There are two main times of the season in this country, the rainy, and the dry season. October through November is the dry season, just after the monsoons. Our group went in December and the weather was a bit colder, but the big advantage that we had was that it was off season and there were very few tourists, both in Katmandu and especially on the trail. During the height of tourism season the trail and the lodges can become very crowded.
The second best season for traveling is from February to April. This is the end of the dry season, but the air isn’t quite as clear. The weather however, is much warmer and the wildflowers are in bloom. In December and January, besides the cold weather, the visibility is good. The plus side to traveling during these two months is cheaper lodging and few tourists. December is the driest month of the year. It did not rain once during our 2.5 week stay.
The rest of the year is not good for traveling. For example, May and early June are quite hot and dusty. From mid June through September the monsoons hit and cover the mountains in clouds, and the roads turn to mud.