My Dr said to do one on day's 19,21,23,25,27 and 29 as my periods were irregular and they we're hoping to catch it.
Went in on day 19 then again on 21 but she checked my results beforehand and they came back positive for ovulation so didn't have to take anymore! Then 2 weeks later I got my bfp!
The first month I had them done on day 21 of a 40 day cycle and it came back negative they are only reliable on day 21 if you have a 28 day cycle. However, when I got my positive I was only 4 do as had Leo had a positive opk.
GL xxx