For most of the country, November-March is (by far) the best time to visit, with temperatures ranging from 40-60 F/5-15 C in the north (excluding Kashmir and Ladakh, which are always cooler) to 65-85 F/19-30 C in the south. March-June is dry and very, very hot (85-110 F/30-44 C), and June-October is monsoon time (20-80 in/50-200 cm of rain will fall in one season). The best times to visit Darjeeling and other mountain areas are the months of March, April, October and November. Obviously, it's less crowded with tourists in India during the off-seasons, but it can be so hot in the spring and summer that it's really not possible to stay outdoors for long periods of time. Another problem is that in the rainy period the monsoon washes away many roads in game parks and the rural areas (south-eastern India has a second rainy period during the cool season). If you're going to India during the spring or summer, you'll want to stick with the hill stations (60-70 F/ 15-21 C). During the winter, avoid Kashmir (30-45 F/0-7 C), unless you're going skiing (and don't even think about Ladakh unless you are going to trek in frozen rivers with Joel - it's one of the coldest inhabited regions on Earth).