I have almost always a 26 day cycle and O each time CD13. On the rather rare occasion I have a 27 day cycle, I sometimes O CD14. But I always seem to O either CD13 or CD14, have never Oed any other cycle day.
It might be useful for you to use OPKs for a few months to see when you O each time and try paying attention to your CM when it starts to look fertile (watery or EWCM).