India's climate varies dramatically. The Himalayas have alpine conditions; Kerala is tropical; Rajasthan is desert. The monsoon (June–September) sweeps across most of the country from south to north — Kerala gets it first in June, the north in July. The monsoon transforms landscapes but limits travel in many regions.
The best time to visit most of India is October–March: cool and dry across the north, pleasant in the south. April–June is extremely hot (45°C+ in Rajasthan). The hill stations of Shimla, Darjeeling, and Coorg offer respite in summer. Kerala's backwaters and Goa are best visited October–February.