In a healthy woman, ovulation occurs

Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The lifespan of corpus luteum is 14 days in case fertilisation does not occur. When corpus luteum degenerates, the progesteron levels go down leading to menstruation. So, ovulation occurs 14 days before menstruation.