Peek date?

Brittany

Hello. So I’m still trying to grasp on how to figure this calendar out sometimes. I been using ovulating kits but my doctor says to stop and just have fun so that’s what we’ve been doing. Just out of curiosity app sent message saying important day to have sex was September 15th ovulation is predicted for September 19th. I’m confused as to why the 15th is important can someone explain? Thanks