Climate plays a key factor in deciding when to visit India. You should keep in mind that climatic conditions in the far north are distinctly different to those of the extreme south.
Generally speaking, India's climate is defined by three seasons - the hot, the wet (monsoon) and the cool, each of which can vary in duration from north to south. The most pleasant time to visit most of the country is during the cooler period of November to around mid-February, although there are marked regional variations.
Apart from the weather, the timing of certain festivals or special events may also influence when you wish to visit India.