If someone is looking for travelling to India, as a tourist then winter season is best time. This is good for both north & south India. This time span compromise from October to March. This is for Rajasthan, Central India & South India i.e. Kerala. In north winters are severe at some places .In south the temperature is pleasant. You will not feel cold at all. In this period humidity is less in south part.
From April, temperature starts to climb up in North India & in South India. But in south humidity also increases which makes things worst for travelling. Even 25 C with humidity will make you as tired as 48 C in Rajasthan.
In north India there is difference in temperature in winter & summer. However, in South & coastal areas the temperature varies from 20 to 40 C with difference of humidity.
In summer only hilly destinations are good options either in North or South India.
In summer good options for travelling are Hill stations either in north or south India. Few famous destinations are Shimla, Manali (Himachal Pradesh), Jammu & Kashmir, Leh Ladahk (North) and Ooty, Munnar, Kodaikanal, Thekkady etc in South.