The calendar based methods, which use averages, include calendar rhythm, standard days, and cycle beads. Although the rhythm of the cycle was known in ancient times, the calendar method you are likely referring to came to prominence in the early 1900s, promoted by several Catholic doctors.
Cycle beads and the standard days methods are recent developments, although I don’t know why anyone would use these! Ugh!
These methods are about 75% effective because they use only averages and not any observations of actual fertility signs.
Modern methods of natural family planning are not based on calendar averages. They are based on observing the actual signs of fertility and charting them. This includes basal body temperature at waking, cervical mucus, and cervical position. These methods are over 99% effective.
The modern methods of NFP include:
Sympto Thermal Method taught by Couple to Couple League and Northwest Family Services. Temperature based charting started in the 1930s.
Billings Ovulation Model, developed by the Dr. John Billings in Australia
Creighton Model Fertility Care developed by Dr. Thomas Hilgers out of Creighton University and based on the Billings Model.
Marquette Model developed by the Marquette University School of Nursing, based on Creighton with the addition of a fertility monitor/ovulation predictor kit (LadyComp, etc).
These methods have nothing to do with a calendar or averages and “irregularity” in cycles is not an issue.