Day 21 is considered to be a marker day when you have a normal menstrual cycle of 28 days. Day 21 is considered to be the middle of the luteal phase. In a normal 28 day menstrual cycle, it takes around 14 days to grow and to develop a follicle, and ovulation of the egg occurs around day 14. The luteal phase begins once the follicle releases the egg and lasts for 14 days. Progesterone rises after ovulation and enters the luteal phase, the second half of the menstrual cycle, and reaches a peak around day 21 in a 28-day menstrual cycle.