I want to point out right away that I am not an anthropologist or an evolutionist, so my answer is not a specialist's answer, but the first answered answer is such nonsense that I simply cannot pass by.
The menstrual cycle is a series of events, and if it is absolutely certain these are two series of events. One of them takes place in the ovaries, the other in the uterus. Both of them are regulated by the hormonal cycle (which could be called the third series of events, but then we risk drowning in unnecessary details, just remember that all the levels of different hormones regulate this).
Uterus: the endometrium, into which there is no there was an implanted zygote (fertilization did not occur) is separated from the uterus and leaves with the blood. Pain in the lower abdomen, for example, is the result of spasms, the uterus, as it were, is trying to throw off the dead endometrium. After menstruation, it begins to gradually grow again, so that by the time of conception (optimal for which is the moment of ovulation in the middle of the cycle), it would be thick, loose and convenient for implantation of a zygote. When fertilization does not occur (hormones signal this) the epithelium, as unnecessary (there is nothing to implant), begins to die off and after a while menstruation occurs again.
Ovaries: almost immediately after the first day of menstruation, the follicles in the ovary begin to ripen, but only the fastest and most sensitive to hormones matures to the end, as a result, after 10-20 days a mature egg comes out of it (ovulation occurs) and goes through the falopian tubes into the uterus, so that there either be fertilized or die.
These cycles located in different tissues have their own speed. Just as the epithelium has a certain rate of growth and death, the follicle has a rate of maturation. Most likely, in the process of evolution, they adapted to each other, because if the egg matured at the moment unsuitable for the implantation of the zygote, then conception would not make sense in it, and apparently in the end, the optimal time to combine them would be 20-35 days. For most women, this is really about 28 days, but it happens in different ways.
Obviously this is due to the peculiarities of the shelf life of the egg. The unlimited life span of an egg is irrational, since many factors can affect its health / integrity, making the latter unsuitable for fertilization. I believe that by trial and error in the process of natural selection, the oocyte was allocated the optimal time, during which it could be successfully depleted and develop into healthy offspring. The rest of the timing options apparently were, but - they could not give a viable offspring and, accordingly, - transfer their genes.