In a regular 28 day menstrual cycle, ovulation usually occurs on about day

Explanation

Ovulation falls near the midpoint of the cycle, roughly fourteen days before the next period begins, because the luteal phase is the fixed part at about fourteen days while the follicular phase varies. Day five is still within menstruation and day twenty one lies in the secretory phase, when the corpus luteum is already active. Counting backwards from the next expected period is the more reliable method in irregular cycles.