Positive OPK?? Calender help!

Hi, I'm still trying to figure all this out. So this app says I have a high chance of pregnancy today but im not supposed to ovulate for another 2 days and if I click on that day that im due to ovulate its says the chance is lower. I thought your chances of pregnancy were the highest on the day you ovulate. Also with it saying my chances are high should I get a positive opk today or would it be tomorrow or the actual date im due to ovulate. Lol this is all pretty confusing.