In the menstrual cycle, what signals the end of one cycle and the beginning of another

A new cycle starts on the first day of a woman's period, typically for 3-8 days. 14 days after the first day of the recent period, ovulation occurs, and the lining builds up. so, ovulation is the signal of the end of one cycle and the beginning of another. hope this helps!! 

When a woman's menstrual cycle starts her breasts may get hard or firm for some women this isn't true this really depends on the individual. I'm not quite sure how you know it's over but coming back but I know how you can tell when it ends completely, you period blood tends to get darker and darker in color when it's about brown your period will stop this is called
meta pause.