The number 24 has many factors, being divisible by 2, 3, 4, 6, 8, and 12. Dividing the year by 4 provides an even division of the year that is analogous to the seasons that are defined by the equinoxes and solstices (spring, summer, autumn, winter). Dividing the year by 8 approximates the human gestational trimester (266 days / (3 * 1.0274913) = 86.3 Martian days). Dividing the year by 8 also approximates a quarter of an Earth year, which is a standard reporting period in accounting and finance (365.25 days / (4 * 1.0274913) = 88.9 Martian days).
The number 28 has several factors, being divisible by 2, 4, 7, and 14. A 28-day month contains exactly 4 seven-day weeks. Divisibility by 7 makes a 7-day-week perpetual calendar easier to construct.
On Earth, the average menstrual cycle is 28 days, and there is at present no reason to expect that this cycle will be substantially different on Mars. Everyone recognizes the importance of the diurnal cycle in regulating human activity. We all need to get a good night’s sleep and eat several meals a day. The other human cycle that is necessary to human existence is the menstrual cycle. Since this is a natural human cycle of time, incorporating this cycle into a Martian calendar is highly desirable. It is only prudent to consider human factors in the design of any system that has a human interface. A calendar is very obviously a system that has a human interface, and the menstrual cycle is very obviously a human factor.
Whether or not something like half the population has a menstrual cycle of something like 28 days is not terribly significant unless they all start and stop essentially at the same time. Otherwise it is just so much noise superimposed over a regular wave.