You should take the climate into consideration when deciding when to holiday in India. The country is generally characterised by three key seasons. The cooler months are between October and February, March to June is hot and dry and then there's the monsoon which generally falls between July and September. Though, being such a vast country, climates and the falling of seasons vary massively across India and you should do some research into the climate of the region that you are visiting before booking. For example, the monsoon in the south-west of India is prohibitively heavy but in Rajasthan is light enough not to be a deciding factor.
India boasts some of the most exuberant festivals in the world and you might choose to coincide your trip with one of these. The biggest festivals are Holi, the Festival of Colours which falls in the spring months and Diwali, the Festival of Lights which takes place in October or November.