Well, on my package it says the best time is between 11am and 8pm, so thats what I try to go by. However, when mine are positive, they are positive early in the morning and late at night. SO I don't know how much it matters.... But I try to do one at 11ish, and usually end up testing again around 7pm. I'm bad and do it twice a day, just in case. I get so anxious!!
The instructions on your package might give you the best time for that brand to be used. Mine was 10-2 and 5-10. I started testing daily at cd10 and started testing twice daily at cd14 (I have a 30 day cycle usually). I can't remember if I got my positive OPK on cd15 or cd16, but I know that I got a positive in the afternoon and around 10 that night. Best wishes!