A fixed climatological year is generally used to determine rainy season onset and cessation. However, due to changes in climate, the fixed climatological year might not be the right basis for the onset and cessation dates estimation. This study proposes the usage of the driest period in the year to establish a flexible climatological year to determine rainy season onset and cessation dates. The driest period of a climatological year is defined as the period of 14 consecutive days, which has the lowest accumulated precipitation. The flexible climatological year begins on the first day of the driest period and ends before the driest period of next year. The onset and cessation dates resulting using this new flexible climatological year are compared against those resulting from the traditional approach. Three onset estimation methods were selected for demonstration of the method: agronomy, anomalous accumulation, and a modified local method. The results showed that overall, the three methods produced similar onsets for both types of climatological years. However, the use of a flexible year showed clear advantages in the application of anomalous accumulation for large and heterogeneous climatic zones because it helped to set a start date and an average daily precipitation, which improved the onset and cessation date calculations.