The calendar method was originally a method invented as a kind of natural contraception to avoid pregnancy. This means that during the woman's cycle one looks at "safe periods "and "unsafe periods". In the safe periods, the chances of pregnancy are non-existent or very low, because during this period there is simply no egg in the woman, which can be fertilized.
The method is alsocalled the Onigo-Knaus method, and it is named after the Japanese Onigo and the Austrian Knaus, who in the 1930s found out when the woman is ovulating and that it is during this period that the woman has the opportunity to become pregnant. (read more about ovulation here: What is ovulation and when is the greatest chance to get pregnant? – Gravidtid
Knaus and Onigo's studies showed that a woman's ovulation occurs 12-16 days before menstruation begins and not 14 days after the last menstruation, as many otherwise believe. But how do you know that it is 12-16 days before the next menstruation? Here the calendar must be in use, and every month one must note when the menstruation came and when it ended. After a few months (officially they say six months) a uniform picture should emerge. If your period turns out to be irregular, then the calendar method can not be used, and it is recommended instead to either contact the doctor or use ovulation tests to find out when you are ovulating: How to test for ovulation? – Gravidtid
When you are looking at the cycle of recent months, count the days between the first day of bleeding. From the cycles where there have been the fewest days between the bleedings. For the sake of example, let's say 28 days, you must subtract 18 days so that you have reached day 10 of the cycle. From the cycles with the longest days between the bleedings - again let's, for example, say 32 days - subtract 11 days. So, day 21. In this example, it will be said that the uncertain period where there is a chance of pregnancy is between day 10 and day 21 of your cycle. This means that it is also during this period that you must go purposefully after having intercourse.
Conversely, if you want to avoid getting pregnant, then these are the days when you should not have sex. However, the method is not entirely safe if one wants to know exactly when one is ovulating. Here it requires ovulation test or temperature method: What is the temperature method BBT and how does it work? – Gravidtid
There are several free apps that can help point out when you are ovulating based on your recent cycles. It may also be that you use ovulation tests or can feel it, but if you know more or less when you ovulate, then it is recommended to go purposefully after having sex the last 4-5 days before ovulation and up to 2 -3 days after. It is during these days when the chance of pregnancy is greatest. Buy our ovulation tests here: Ægløsningstests – Gravidtid See our thermometers here: Termometre – Gravidtid
Sources: Sundhed.dk article by Erik Fangel Poulsen, specialist in gynecology and childbirth,netdoktor.dk