Doctors like to say that in a 28 day cycle, ovulation usually occurs on day 14. In longer cycles, ovulation occurs 14 days before the start of mensturation, however 14 is just the average. It could vary between 10 and 16 days.
I get delayed ovulation occasionally due to stress, travelling, or illness. It makes it really hard to predict ovulation using just a calendar. I was supposed to OV on May 22nd, but it ended up being a little over a week later. In May we had travelled overseas which is a well known cause of late ovulation.