I need help. Confused af

My boyfriend and I are planning to have sex next weekend and I'm trying to avoid pregnancy by knowing when I'm ovulating or not because I know if I were to have sex when I'm ovulating, it would be likely for me to get pregnant (correct me if I'm wrong please) even with protection.  I feel like I shouldn't depend so much on app because I don't think apps like that aren't always right. How can you know that you are ovulating without an app?