Traveling to Kullu Manali is possible throughout the year. But it all depends on your requirement as what exactly you are looking for your Holidays.

If you are keen to see snowfall and enjoy some snow activities with your family and friends, November to March is the best time to visit Manali. Some tourist places in Manali like Rohtang Pass (and sometimes Solang Valley too) gets disconnected from the city.

You cannot enjoy some adventure activities like River Rafting. Water is so chilled that you cannot get into it for a single minute.

If you want to enjoy The actual scenic beauty, Greenery all over, August to November mid is the best time. You can also visit Rohtang Pass during this period.
Apr to July mid is the season time – Too much crowd and traffic, so no comments (Personal Choice).

July and Aug is a Rainy season, Though good again to enjoy greenery with rains.