When to start OPK after missed period?

Claire 💐

AF is 7 Days late. I have taken 3 pregnancy tests and received 3 BFN including one this morning. AF being lates isn’t a huge difference in my norm BUT I have been regular to the exact same day for the last 3 cycles. I have never used OPKs and I am wondering when I should give it a try? I don’t want to miss my chance if I still haven’t ovulated yet... help please 😳