My period varied between 'sort of' and it's a constant surprise'. I can figure the general time based on the date of the last one, but it could still be another week or two away. Sometimes my body is very clear that it's about to show up, other times there are no signs whatsoever. I think three times in my life it has appeared on the exact date as the previous month, which was kinda neat. I do try and track it though, as far as I can.