The calendar method is a natural family planning technique used to predict fertile days in a woman's menstrual cycle. By tracking the cycle length over several months, individuals can identify patterns and estimate when ovulation occurs, which is typically around 14 days before the next period starts. This method helps couples plan or avoid pregnancy based on their understanding of fertility. To use the calendar method effectively, one must record the start date of each menstrual cycle for at least six months. By calculating the shortest and longest cycles, individuals can determine the fertile window, which usually spans a few days before and after ovulation. This approach requires careful monitoring and consistency for accurate results.