I know that your body makes LH all the time, so the manufacturers try to make the strips so that you can see when it's building up towards ovulation. So you're likely to have a few with no line, then some with faint lines, and finally (hopefully) some with REALLY dark lines. Those are your peak readings (+OPKs) and they indicate that ovulation is imminent in 12-36 hours. Then after a day (or two or three) of very dark lines, you'll get another fainter day.
Q: Does that mean that the first day of negative OPKs after a positive OPK is the actual day of ovulation?
A: Ovulation does not necessarily happen after the OPK turns negative. It could happen, but that's not the rule. What matter solely is when it OPK FIRST turns positive not when it eventually becomes negative. Ovulation happens within12-36 hours after the OPK FIRST becomes positive. What happens afterwards is not important. It could be positive for a day only or for 3-4 days. The length of it being positive doesn't predict when ovulation happens.