“an unmarried woman ... would be sure to know when her next period was due”
I never did. Many women are extremely irregular.
LOL, well...irregularity is one thing...being caught by “surprise” is something else.
I would think a woman would want to know around the time, give or take a few days, when she was due.
Some women who are very regular give a day or two of leeway, where those who are very irregular may give a week or two...:)
Being a guy, this was generally something opaque to a large degree, unless...it wasn’t!