Travel Season in Nepal are Autumn and Spring Season on which time weather is mostly fine and clear. Climatic factors are very important in deciding when to visit Nepal. October-November, the start of the dry season, is in many ways the best time of year: the weather is balmy, the air is clean, visibility is perfect, and the country is lush following the monsoon. February-April, the tail end of the dry season is the second-best period: visibility is not so good because of dust, but the weather is warm and many of Nepal's wonderful wild flowers are in bloom.
In December and January the climate and visibility are good but it can be cold: trekkers need to be well prepared for snow, and cheaper hotels in Kathmandu - where heating is nonexistent - can be extremely cold in the evening. The rest of the year is fairly unpleasant for traveling: May and early June are generally too hot and dusty for comfort, and the monsoon from mid-June to September obscures the mountains in cloud and turns trails and roads to mud.
* Spring: March - May
* Summer: June - August
* Autumn: September - November
* Winter: December - February
The weather in Nepal not only depends on the time of year, but also on the altitude of he places you are visiting. The width of Nepal is only about 200km on average, but within this short distance, the altitude of the land ranges ises from lowly 60 meters all the way up to above 8000 meters, and temperature change accordingly. Kathmandu is at 1350 meters. So, what to wear? Cotton wear will be a good choice for anytime of the year in Kathmandu Valley.
It is recommended that you carry light or medium-weight clothes that are easy to wash. Between October to February, woolen sweaters, jackets or similar other warm outfits are necessary. From March to May, light clothing such as short-sleeved tee shirts and shorts will do perfectly fine in Kathmandu, Pokhara and most other towns.
For mornings and evenings, a jacket or heavy woolen sweater (you can find beautiful ones in reasonable price in Kathmandu) will be essential. For months from June to August, it is recommended that you bring an umbrella or raincoat and a pair of sandals with you as these months are the rainy months of Nepal. Expect lot of walking even if you don't plan to trek. It is recommended that you bring comfortable footwear: sneakers and sandals are the best.