Actually you can go any month.But If you are going in oct-jan,;you will surely get snowfall.But you cannt go to Rohtang Pass in this time;because it get closed due to snowfall. So for the Rohtang pass you should go in summer months. I have done Skiing,Rafting and paragliding there (MuSToDoIfYouAreGoingToManali).
Hey sachin!! Being from Himachal only I recommend you not to visit manali as it is overrated and overcrowded in winters. Instead you can explore the less commercialized places like Tosh, Kasol, Kheerganga, Manikaran etc. in Kullu valley. Or you can visit Vashisht, Jogini falls, Solang valley etc.
Hello Sachin, It is great place to visit and favourite of almost all! You can visit any season or month of the year and you will feel refreshing but if you want to see snowfall you should visit between December and January. You can plan your trip accordingly, depends on number of the days you have to travel.