Hello, the change to wet fertile type mucus (ie EWCM) signals that O is on it's way and it can be gradual or sudden. Your peak fertile time is considered to be the last day of this type of mucus before a return to the non fertile tacky type.

According to Francesca Naish, in her book, "Natural Fertility" p 61 - "[The last day of fertile type mucus] ...can coincide with ovulation within 1 day (85 per cent of women) or 2 (a further 10 per cent, making 95 per cent of women)....The peak symptom, the last day of fertile mucus, is only recognisable retrospectively, after a definite change to dry days or infertile mucus, which occurs as a result of the increase in progesterone."

So basically, when you see EWCM you know ovulation is likely to be in there somewhere but you can't really pin point it until after the mucus has changed. I think this is why a lot of the ladies in here use multiple methods, ie temping, maybe baby and OPKs to try to narrow it down, because no single observation is going to tell you with complete accuracy when you ovulate!

I hope that helps, I recommend Francesca's book (prob at your local library) even tho it's about natural contraception as it has the most in-depth info about CM observations, I've found it very useful!
:-)