I only know because mine have always been irregular, therefore it is in my interest to be alert to signs that say “Kind of a fortnight hence”. I certainly couldn’t say “I will be menstruating on day X next month.” I do keep a record as well, but only because all of this together allows me to start taking the NSAIDs a couple of days in advance (blessed, blessed NSAIDs), and I don't have any PMS symptoms that would otherwise allow me to anticipate.

Also, and now I am going to sound like a complete nutter, I am convinced that I can exert a degree of mental control when really necessary to ensure that though my period is likely to turn up on day 1 of my skiing holiday, in fact it waits until the day afterwards. This is too reliable a phenomena to put down to complete chance.