When is the best time to go to Nepal?

You can find good places to trek somewhere in the Himalayas at anytime of year. Even though weather is probably the best guide for deciding when to plan your trip to Nepal.
The width of Nepal is only about 200 km on average, but within this short distance the altitude of the land rises from lowly 60m to all the way up to above 8000m. Hence the weather depends upon the altitude of the place in Nepal. However, in general Nepal has four climatic seasons:
Autumn: September, October and November are considered the best times of the year. During this period, the climate is dry offer the clearest skies, least rain, the festivities of harvest season and mild with temperature fluctuating between 20* C to 30* C (68* F to 86* F). The monsoon will have just ended, and clear skies with optimal temperature will prevail. The main festivals of Dashain and Tihar (Hindu equivalent of Christmas in terms of festivity) fall during these months. However, this is also the busiest tourist season, and the main tourist centers and trekking trails tend to be crowded with travelers like you.
Winter: Winter starts from December to February are cool and mostly clear. The morning and evening are very cold while the afternoon is pretty sunny. The temperature during these months rises from 15* C to 20* C (59* F to 68* F). The tourist flow ebbs a little, but not significantly, between the winter months of December and mid-February. It catches up once again between mid-February and mid-April.
Spring: Spring starts from March to May are warm months, though it is often hazy. The temperature of this season fluctuates between 20* C to 30* C (68* F to 86* F). Rhododendrons blooms at ascending altitudes from February through May are also a very good time to trek as snow melts at the upper altitudes.
Summer: Summer starts from June to August. These are also the pre-monsoon and monsoon months with occasional evening-thunderstorms and hot temperature, which time it rains almost everyday and most of the Himalayas are hidden behind the clouds. June is warm and humid, though still a good time to trek to the Everest area. July and August are monsoon months with lots of rain, but still you have great chance to trekking behind the Himalayas Manang, Mustang and Dolpo known as the rain shadow areas of Nepal.
September sees the end of the monsoon. The whole country is beautifully green and the trekking season starts again. In short, plan to visit Nepal between September and May; keeping in mind that September - November and March - May are the best times (but crowded with other travelers).For detail information on temperature and rainfall, check the Weather Chart.