Generally, the best time to trek is the dry season from October to May. It is difficult to trek in the monsoon season from June to September due to the slippery trails and the rising and unpredictable rivers. However, you could consider trekking in the rain shadow areas of Upper Mustang, Upper Dolpa where it does not rain as much as it does in other parts of the country.
October and November are the best time for trekking as the weather is cool and the atmosphere fresh. December, January and February are still good months for trekking but it is very cold at high altitude. From March the weather starts to get warmer with the weather being hot and humid at the end of May. You could enjoy the splendid views of rhododendrons in the hilly areas during this period.