In the month of December the temperature usually varies between the mid 20s (Celsius) and the high teens. Rain is extremely unusual.
Detailed instructions are available from the home page of the Department of Theoretical Physics.
Currency exchange is not available on campus. Thomas Cook or American Express are about 20 minutes away by taxi or public transport. See current exchange rates for the Indian Rupee (INR).
Accommodation will be on campus or within walking distance. Details will be emailed to you before the meeting. Breakfast is included in the guest house charge. Lunch and dinner are available in the canteens on campus. If you want to eat out you could consult a guide to restaurants in Mumbai.