According to my experience :February and March is the best time to visit Kullu Manali . If you want to encounter first snow fall you must visit in early December .
Hey. The best time to visit Kullu, Manali and Shimla is during the summers, from the month of April to June. During the summers, the snow melts, offering lush green meadows, bright mornings and opened trekking routes. In fact, March is also a decent time to visit these 3 destinations. It's suggested to avoid visiting these places during June, as a lot of tourists visit these places during the month of June and that leads to traffic jam and surged prices. It is suggested to not visit Kullu,Manali and Shimla during the monsoon either, as the months of July and August witness landslides. During the autumn, the weather turns cold. But if you are willing to endure chilly nights in exchange of off season perks such as cheap hotels, inexpensive taxis and less tourists, you can visit Himachal Pradesh during September and October. From November, the weather turns extremely cold, and the state starts receiving snowfall. If you'd like to see snowy routes and enjoy challenging treks across the snow-laden trails, you can visit Kullu, Manali and Shimla from the month of November to February.
I recently visited kullu Manali and didn't find any snow but still can't describe how beautiful the place was. If you are visiting this place for snow then summers are best but summers are peak times so there might be traffic issues and we need to book hotels prior a week or two .