Here is basic information on how the menstrual cycle & ovulation occur. http://womhealth.org.au/conditions-and-treatments/understanding-your-menstrual-cycle-fact-sheet
With that said, I charted my cycles for years & was well aware of how my body worked overall. I have actually taken education on the use of charting to avoid pregnancy. It worked very well for me for a decade & then one time I ovulated on day 4 (which for most women would be during menstruation) and that is how I got pregnant with one of my children.
So there is no time in a cycle you can have unprotected sex and be absolutely certain you will not ovulate, even if you have always ovulated on a predictable schedule. I did know this. It was okay with me overall to have that unplanned pregnancy, hence why I was comfortable taking that risk as a married person who had children already. If you do not want a child, wear a condom, every time. It really is what you need to do. You cannot know for sure what a body will ever do because it is impacted by stress, sleep, foods, etc and hormones - things you cannot always account for. Even when a woman takes hormonal birth control, there is a failure rate. None of the birth control options other than sterilization have close to a 0% chance & anyone telling you that is not being factual.