While it would be difficult to give you an exact month, but usually the earlier you book your flight, the cheaper it will be. Since you are going in September, now would be a good time to book. To make sure you are getting the best deal, you can use Skyscanner's "whole month" feature to figure out which day works out the cheapest in September. Here's a link: https://www.skyscanner.co.in/?associateid=PCP_FLI_12454_00001&associateid_proc=true
I hope you have made your trip. It would be helpful for travellers if you share the travel details:
5.Places to see
6.Tips for travellers etc.