The tropics of East Africa have two distinct rainy seasons; the Short Rains in November and December and the Long Rains from March to April. These usually occur when they are supposed to, but not always!
Serengeti National Park has the largest concentration of migratory animals in the world. There could be as many as 1.3 million wildebeest, 200,000 zebra and even more Thompson's gazelle, plus about 20,000 buffalo. There are 3000 lions in the Serengeti.