In agroecological zone 5 of Benin, agriculture is mainly rainfed and is closely linked to the evolution of the characteristics of the rainy season. This is intended to be a scientific contribution to the determination of the annual agricultural period linked to the rainy season at the scale of agroecological zone 5 of Benin. A reliable prediction of the start and end date of the rainy season would effectively help the preparation of fields, the mobilization of seeds, labor and equipment and also reduce the risk of sow too early or too late. The model of the cumulative percentage of average rainfall and the hydrological criterion, methods based on the analysis of rainfall at the scale of the study area, made it possible to identify the periods determining the rainy season in the study area. We observe greater instabilities at the start of the seasons than at the end. This trend differs according to climatic regimes.