I think it was when the old Johnathan was explaining to Jotaro in part 3 (season 2) about the threat Dio posed, while mentioning(retconning) the star on his body via a spirit photograph. I think that was somehow linked to why Jotaro's mom got a stand that was slowly killing her... they were awakened to their bloodline or something, hence, the search for Dio began. Again, if I remember right.