India being a vast country, the temperatures differ from one region to another.
They vary between 30°C and 40°C during the hot season or exceed 45°C on the central part. However, if you plan to spend several days in the northern mountains, it can be freezing cold ! Do not forget to bring one or more warm clothes.
The monsoon raging in India starts from late June to mid-October. Nevertheless, India is a destination at any time of the year. Simply expect less rain and more sun on the beaches. However, there will always be a part of the country where the weather is not going to change.
It is better to travel to India between November and March when you can fully enjoy the country. It is the best time to spend a few days off the beaches of Goa: the water is clear for diving and it is particularly sunny to enjoy the beaches. The highlands are rather clear even if there is a risk of rainfall.
If you wish to visit the Far North region, specially Ladakh, the ideal time is from May to October.