What marks the end of one menstrual cycle and beginning of another?

The menstrual cycle begins Day 1 of your period, about half way through the cycle you ovulate and the last day is the day before your next period starts. Day 1 begins the cycle all over again. The last day of your cycle tells you the length of your cycle, on average between 24 and 34 days, although longer and shorter cycles are still "normal".