That used to be known as the rhythm method, aka Vatican roulette.

No, it didn't. Entirely different methods. One involves bodily awareness, tracking symptoms of fertility, etc, basically all the things they tell couples who are trying to conceive, except with a "don't" in front of "have sex on these days." The other involved a calendar. Note: I am not advocating this particular method, and I think any couple who isn't prepared for a child should use another form of contraception, but , well, much like with fertility science, pedant.