The first three months of the dry season September, October and November offers the beast of the views. These are also the very months where the trails are overcrowded by huge amount of trekkers from around the world. The views are usually crystal clear, the weather is warm during the days and it is not very cold at night.
December to February are as well considered as good months to trek to lower altitudes and up to Everest base camp. However, passes like Thorung La, passes in the Everest, and treks like Dolpo, upper Mustang , Makalu base camp and Humla usually gets closed because of heavy snow fall. On the other hand, the trek in the Annapurna and up to Everest base camp is beautiful in this months and avoids traffic.
It is the almost the end of dry season in the months of March, April and May. The prolonged dry season results in dust, which further contributes to not so beautiful views as September, October and November. However, this is considered as one of the best seasons in the year. On top, it is spring at this time of the year and along with views of snow clad mountains, one can be mesmerised with beautiful blossoms of the wild.
The months of June, July, August and mid September is considered as monsoon period in Nepal. The views are obstructed and is mostly cloudy and on top the trails are slippery and full of leeches. However, this season is perfect for trekking in Mustang, Dolpo, Humla and Jumla.