Ovulation occurs when hormone changes trigger your ovaries to release an egg – usually, 12 to 16 days before your next period starts. You are most fertile on the few days leading up to and including the day of ovulation. IT occurs about 24 to 36 hours after the Luteinizing Hormone surge, and you are at your most fertile on the day of the LH surge and the day after. It is essential for fertility and childbirth.