Once you've ovulated the egg will only be fertilisable for somewhere between 12 and 48 hours (or thereabouts). So sex before (cos the swimmers can lurk for a few days), at ov or very shortly after ov can get you pregnant. Once that window has passed, the egg can't be fertilised.
They show positive 12 to 36 hours before ovulation. As soon as you get a positive is the prime time, and the next day just in case. Ideally also in the couple of days before the positive as the sperm can survive a few days. On the second day after the positive it is probably too late.