Being equatorial, East Africa is not subject to the 'seasons' experienced further north and south of the Tropics.
Instead we have two 'rainy' seasons influenced by the monsoon winds of the Indian Ocean and movement of the Inter Tropical Convergence Zone (ITCZ) north and south across the equator.
The weather pattern of Western Kenya including the Masai Mara is heavily influenced by Lake Vicotria and has a much higher annual rainfall than the the East African average.
The 'long' rains begin around early April and last until the end of May.
June, July and August are relatively cool dry months, and the time of year when the wildebeest migration reaches the Mara which generally remains until October.
The 'short' rains begin around mid-November through to December.
The warmest time of year is after the short rains from January to March.
During the drier months, the grass is shorter allowing for easier wildlife viewing. Seasonal pools and rivers dry up forcing animals to concentrate around permanent water sources such as the Ewaso Nyiro River in the Northern Frontier District. Generally this means game is more concentrated in such areas and therefore easier to find.
During the wet months, seasonal water sources fill up, allowing for the wildlife to diaperse further afield, coupled with the longer grass this means that wildlife viewing is not so easy. The upside however is that during the rainy seasons, the land comes to life, regenerating with the flora turning lush, and fauna thriving on the newly abundant sources of food and water.
Aside from the general weather pattern in East Africa, due to a vast range in altitude and habitat, micro climates can vary greatly at the same time of year. For example, montane forests of Mount Kenya can be close to freezing on a crisp July morning, yet 80 kilometres away on the lower plains of Samburu, the temperature could reach over 30 degrees celsius.
Even during the warmer times of year, once the sun sets, evenings can get cold. It's therefore advisable to have a selection of clothing 'layers' to add or decrease as the temperature changes, most noticeably around sunrise and sunset.