Anytime is good time to travel but make sure you are prepared for what rainy season in mountains can bring with it. Buses may get late. Travel by Himachal Road Transport Government Buses as they have the best drivers. Also from my experience Kasol looks blissfully beautiful when it rains. Happy travelling!
October to June are the best months to visit Kasol, the 'Amsterdam of India'. The best time to visit Tosh is from April to October, though it has a pleasant weather throughout the year.