The best season to visit Nepal and Everest region is after the monsoons that end in August, and before the winter sets in. The months between September and December are the most preferred ones by the visitors. The rains wash the dusty tracks and the valley looks magnificent with blooming flowers. Post Monsoon Nepal welcomes you with flower-laden plains and brimming rivers. It is humid in plains at that time but it is still better than the scorching heat of the summers.
It is best to visit hilly areas in summers; that is, the month of May and June. The average temperature at that time hovers around 22-25 Celsius. Nights are a bit chilly in the lower Himalayan region but are comfortable. Mountainous areas are extremely unsafe in the Monsoons. The persistent rains make the hilly tracts slippery. The landslides are very common in the monsoons. Again in the post-monsoon months, you can start for the hill tours.
In winters, most of the hilly areas become out of bound due to very heavy snowfall and avalanches. But if you are fond of snowfall then you can opt for lower Himalayan regions that look beautiful in the blankets of snows.