The date you get the keys. In a new project it can be months after the promised date; the RERA page shows the committed date.
How it works and what to check
In a RERA-registered project, the completion date the builder declared on registration is on the state RERA website, and the agreement for sale must state the possession date. If the builder delays, the buyer can usually claim interest for each month of delay or, in some cases, withdraw with a refund and interest.
Possession should follow the occupancy certificate. At handover you get a possession letter, inspect the flat, and start paying maintenance.
Under RERA you can generally claim interest for the delay while staying in the project, or withdraw and seek a refund with interest. File the complaint with the state RERA authority.
Where do I check the promised possession date?
In your agreement for sale and on the project's page on the state RERA website.