How do you know which day you are ovulating?

I'm wondering because I'm trying to figure out which day is best to take a pregnancy test. I've seen women on here say that they got a bfp 8 or 9 days past ovulation. On this app there are like 5 green days for me so do I start counting after the last green day? Or would I be ovulating on the day that has the highest percent chance of me getting pregnant? Which is 33%. The day after that is 22% then the next is only 8%, but still a green day. Thanks! 😊