When would it be considered day of ovulation?

I would say you’re at your peak. I’d try tonight and again tomorrow on DPO. You will get the hang of it once you track a little longer! You could also try every other day during your window!

Usually the highest one is peak

Everyone is different but ovulation usually occurs 24-36 hours after the surge. So if CD 18 is your highest lh then I’d say CD 19 (maybe 20) would likely be 1DPO.