Crop calendars, indicating the optimal time of planting and the time of harvest, can aid both farmers and advisory experts in the field of agriculture to reduce any risks. Existing crop calendars are made available for many countries but the planting and harvest periods are very general and can even comprise a period of two months. Defining the onset of the rainy season for the different regions and providing more detailed information about crop cultivation in relation to the length of the growing season, will help farmers reduce their losses and increase crop yield. This report describex a methodology for building a crop calendar of high quality, to better inform local farmers and advisory services in this field. The first part of this manual describes the methodology in three sections, so that this can be replicated for other countries with a distinct rainy season. The second part consists of a case study for Senegal, where this methodology has been tested and crop calendars were created as an output. Rainfall data for this country was provided by ANACIM, who were also involved in this project. This work could not have been completed without the support provided by Agence Nationale de l'Aviation Civile et de la Météorologie/ National Agency of Civil Aviation and Meteorology (ANACIM). Through a collaboration where rainfall data, knowledge and information were exchanged, a qualitative analysis of the methodology could be executed for Senegal. In addition, this work was funded by the Food and Agricultural Organization of the United Nations (FAO) under a joint WMO and FAO project. WMO would like to graciously acknowledge the continued collaboration and support of FAO on issues related to agricultural meteorology.