Ovulation Question!

I took a opk last night and it was positive but to make sure it was a definite positive I took another tonight and it was an obvious negative. So the question is: if the opk is negative tonight does that mean I ovulated yesterday? Or did I ovulate earlier today? I'm just not sure if I should call yesterday (CD 15) my ovulation day or today (CD 16) my ovulation day? Please help me - this is my first month ttc and if I don't conceive this month I'd really like to have a rough idea of when I'll ovulate during next month's fertile week.