Confused!!

Maria
Hello beautiful ladies :) 
Me and my husband decided to have a baby and we started actively trying this last week. Since I was a bit confused with the ovulation days of my cycle, although I have a very stable cycle, I went ahead and bought some ovulation test strips and tested for the last days to understand which day is really my ovulation day and also to maximize our chances. 
These are all my strips, as you can see the second line was really obvious in cd 13 but also today cd 15? Which one would be my ovulation day? 
Thank you so much :)