The wet (monsoon) season from June to mid of September bring the sparkling clean, green and lush with great views around. Autumn (September to November) and spring (March to May) bring almost perfect weather and are definitely the best times to come to Nepal. There are some great festivals to enjoy in autumn season, i.e Dasain and Dipawali colorful festivals.
Spring and Autumn are the best times for visiting Nepal, since the weather tends to be moderate and stable, with clear skies. However, due to the complex nature of Nepal’s geography and climate, there's a lot more to this. An ever-increasing number of travelers visit Nepal in winter and summer, and for good reason. With the right information, travel plan, and preparation, Nepal is a great year-round destination.
Nepal has tremendous variation in climates as there is a vast variation in altitude. Trek in Nepal season generally known best as two seasons after the monsoon and after the winter. Mid of September to mid of December consider ever the best peak up season for trekking in Himalayas and travel around Nepal. During the period of autumn season, weather is pleasant, not too hot and cold, its crystal clear. There is another best season of trekking in Nepal after winter when sun getting more warmer in spring time may trekkers to Himalayas can again enjoy comfortable climate condition. In spring season in Nepal many different types of flowers bloom both trees and ground and it is also the best time to climb up peaks for expedition lovers.
Spring season starts from beginning of MARCH and it runs to the end of MAY. Spring season is the second peak up season after autumn season end up. Traveling or trek in Himalayas in spring season is the best time if you expect not much trekkers on trekking trails compared to Autumn season.
In spring you will see flowers in bloom especially rhododendron (known as Laliguras locally) the National flower of Nepal. We can see rhododendron flower in mid-march to mid of May depending on altitude and aspect of slope. Spring season (March to May) tend to be warmer, clear mornings, crystal clear skies in day and at nights. But we never know, some time we get balmy stormy bad weather at any time of years. Trek in spring is really a good time to see blooming varieties of wild flowers, mild warm at lover elevation but cold in higher elevation, beautiful greenery landscape with clear mountain views.
Autumn season starts in early October and carries on to end of November. It is the best peak up season in Nepal. We can see clear skies best for mountain photography trip. Monsoon rain ( June to mid of September ) clears out the dust and pollution from the air and it is normal to get a blocking high pressure sitting over the mountain resulting in sunny weather and crystal clear skies. It is possible in autumn to get sunny conditions with blue skies throughout the day whereas this is unlikely in spring when cloud normally builds up in afternoon. Trek in Autumn season is the best time in Himalayas. You can experience the better visibility of snow caps mountains, excellent clear weather, mild warm day and clod nights, safe and excellent trekking can be experienced and we can experience the Nepali local festival like Dashain and Tihar in this season.
It is also possible to travel around / trek in Nepal during winter as well from December to February and Monsson season. Some years the monsoonal influence continues into the first week of October and other years it winds down in early September.
It is also possible to visit Nepal during the monsoon months from July to September however this is certainly not the best time of year to visit Nepal and trek to Himalayas but summer is the best for botanists and researchers in mountain. Different places of Nepal like Manang , Mustang and Dolpo are consider as a rain shadow area so summer is the best time to trek in that area. It will be neither cold nor hot in rain.
Winter season is not that bad to vist Nepal as usually sunny and stable weather conditions if you are travelling around December and January. The temperature would be too cold for high mountain trek or peak climbing but trek like low altitude still wonderful time to explore too. High altitude weather are freezing cold with mass of snow fall. Trek to high altitude can be challenging because trails covered up with full of snow. But in lower altitude below 3000m from sea level would be no problem at all.