The first one's were way back on cd 12 and then the middle darker ones were cd 15 and 16. The most recent one was cd 18. I'm on cd 19 today and this mornings was a bit lighter in my eyes, but it was smu and quite diluted. I have a 32/33 day cycle usually, but it's been known to be shorter up to 30 days.
I wasn't able to test on cd 13 and 14 and am afraid I missed the surge then. Especially since I'm on cd 19 and it still isn't any darker...