Mid September to November is the ideal time to visit Nepal. The landscape is green and lush and Nepal is gorgeous. The air is clear giving way to beautiful Himalayan vistas. The weather is not too hot nor too cold, it is peak tourist season and it is also the main season for Nepalese festivals.
December to February can be a chilly but beautiful time of the year to trek. The climate and visibility are still good, excellent clear views are common at high altitude but it can get very cold. Be prepared to encounter snow on the high altitudes and some of the higher passes may be closed.
March to Mid May would be the second best time to trek in Nepal. The visibility is not quite as clear as earlier in the dry season. Mornings are usually clear, with clouds building up during the afternoon and disappearing at night, to reveal spectacular starry skies. As the season progresses, the magnificent rhododendrons bloom at higher and higher altitudes until the flowers reach tree line. In mid-May the weather warms up and many species of plant and animal life emerge, rarely seen at other times. May is also the climbing season in Nepal so don’t be surprised if you see an Everest summit team in town.
June to August is the warm and wet monsoon season. These months are great for plant lover as the higher valleys and meadows blossom with flowers and lush vegetation.