I started late, with menarche at 2 weeks shy of 16. First I was irregular - either 35 days or 21 days between periods. But on average, always 28. Than, within a year, the menses became regular, exactly 28 days, and to the time of day. I know it can seem surprising: I was once on a camping trip with my bff and we were going to cross marshlands next day. I told her: oh well, tomorrow afternoon (the hour) my period will start right when we will be in the marshes, and she was "How do you know?"

I knew from past patterns.