Ovulation test questions

So I have never bought one and I am thinking if doing so. 4 apps tell me I should ovulate the 6th or 7th but with the holidays and family staying over I have not been consistent with my temperature readings so I thought maybe this month to try one. Do the tests tell you when you are about to ovulate or when you do? I thought the best chances for pregnancy are the day or two before ovulation. If it's testing when you did ovulate, didn't you already miss the best window?