There is no exact time if you want to be in Nepal, you can come here at any time of the year and get the best of it. When it comes to trekking and adventure there are two prime seasons, the first one is at the beginning of the year (March-May) and the other towards the end of the year (mid September-November). If you are into flowers and blossoms and want to see patches of green along with the snow clad mountain you will surely indulge in spring whereas if you are into fall color you will rejoice the season at the end of the year.