The peak tourist season begins in cool September. Autumn (late September-November) and spring (February-April) are the best seasons in Nepal. Pleasant breezes sweep the plains and while autumn brings the hectic festival season, spring (February-April) brings the bloom of flowers to the countryside.
The best time for trekking is between October and May, when the weather is relatively stable. In Nepal, when it rains it pours, and the months of slush from June to August are distinctly avoidable. May in the terai is hot, almost like the northern plains in India. But for a taste of local festivity and ritual fervour, the time to visit must be Dasain. Ten days in end-September or beginning-October are devoted to ‘puja’ time. The goddess Durga (incarnation of Shakti or Power) is celebrated, venerated and worshipped. All banks, government offices, and anything that should be working all year round shuts down for a blissful vacation. Tickets on buses and planes get booked out of the reckoning so wherever you get to during these days, be prepared to stay put!