In the Savanna there is two seasons. May until November is the rainy season. On the graph you can see that the amount of precipitation is higher on average during that season. October until March/April is the dry season. As shown on the graph, the precipitation is lower on average during that time (http://www.blueplanetbiomes.org/african_savanna.htm). Most animals migrate during the dry season due to most plants and water sources drying up. All the animals migrate back during the wet season due to all the plant and water sources being replenished.