What process signals the end of one cycle and the beginning of another in the menstrual cycle

The menstrual cycle is the set of physiological phenomena, occurring most often periodically (approximately one month), which prepare the body of the woman for a possible fertilization.
The process signal of the end of one cycle and the beginning of a new one is menstruation which is explained by the release of the unfertilized ovule with the placenta precursor (which is a blood).
In the case of fertilization, the menstrual cycles are "paused", and "resumed" after the childbirth.

No because the eaeht is on a 23.5 degree axis tilt and so when it is winter the northern hemisphere is pointing away from the sun and the soutubern hemisphere is pointing toward the sun