Dear Yogesh

Inorder to do the Phd in the elite institutes like IIT and NIT'S  you need to be stick with their official website for the notifications for that particular college because there is not any fixed date / month / time in which they will release their notifications for PhD usually. So inorder to do that you have to daily visit the official website of that respective college for the further updates regarding the admission in PhD course.

http://www.iitb.ac.in/en/education/admission-notice-pgphd-programmes-2018-19

All the best