India is a tropical country. Nevertheless, there are huge variations according to the region and the season. The coolest months are from mid-November to mid-March, which also happens to be the tourist season. In the south, and on the coasts, day time temperatures even in the cool months can be in be the mid-20s centigrade, though the nights are cool. In central India, and in the hills in the southern part of the country, night temperatures can drop to under 10°C in winter. In the north, winter temperatures can approach 0°C even in the plains, and of course it drops below freezing in the Himalayan region. Summers are very hot, with some parts of south and central India, and the plains in the north, getting temperatures over 40°C. The monsoons, the rainy season, stretch from June until October, with different levels of intensity in different parts of the country. The West Coast and the north-east get the heaviest rain (two towns in Meghalaya vie for the title of the place with the most rainfall in the world).
Winter - In the south, in the hills, you may need a light jacket in the evenings and early morning, or on overcast days. In the north, you may need to dress warmer, with light woollens even during the day. Consider dressing in layers and carrying a small day-pack to stow away some clothes as the day gets hotter.
Summer - Light cotton tropical clothing, sun hats or caps, and sunglasses are recommended. For those travelling in the hills or mountainous areas light woollens may be needed for the nights.
Monsoon - Light, quick-drying clothing, and either a raincoat or an umbrella or both (especially in places where there is a heavy monsoon).