We 'Homo Sapiens' have long been living in a dual reality. On one hand is the objective reality of rivers, trees and mountains; and on the other hand is the imagined reality of Gods, Nations and Corporations. In Short, Visit Kasol before it becomes a concrete jungle inhabited by the species of Homo Sapiens.
For now, Mountains and Rivers rule Kasol: the mighty valley of stoners.
The best time to visit Kasol is the time you choose so beware of anyone telling you otherwise. But yes, if you want to go on treks, you should refer to a good websites like IndiaHikes or Trek Himalayas. I still remember the saying by Tolkien, “Not all those who wander are lost”. This is totally not true for Kasol for, “All those who wander here are lost”.