The rhythm/calendar method is a natural method of family planning which relies on calculating a woman's safe days-these are days in a woman's cycle when she's less likely to get pregnant as the egg is not yet released from the ovaries. Moon beads/calendar charts can be used to calculate safe days. Calculating safe days is NOT a reliable way to prevent pregnancy because women's cycles can change each month (due to stress, hormones, diet, etc).Even if her cycle is regular it's possible to get pregnant throughout the month. Considering the risk of HIV and Sexually Transmitted Infections (STIs) there are NO SAFE DAYS at all. Modern methods are MUCH more reliable.
Attribution: Marie Stopes