Map from Google - Masai_Mara

In the Masai Mara National Park, located in southwest Kenya, the climate is pleasantly warm, with cool nights, all year round. The area is located just south of the Equator, but at an altitude between 1,500 and 1,900 metres (4,900 and 6,200 ft). The temperatures are slightly higher from October to March, while they are slightly cooler from June to August. At night it can be a little cold, and the temperature can drop below 10 °C (50 °F), especially from June to August.
Here are the average temperatures.
Average temperatures - Masai Mara
Masai MaraJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)121212131312111112121212
Max (°C)282827262525252527282727
Min (°F)545454555554525254545454
Max (°F)828281797777777781828181

Rainfall amounts to about 1,400 millimetres (55 inches) per year. The wettest month is April, the driest is July. In the two rainy seasons (see below), especially in April, the rains are abundant and can cause flooding.
To the south, across the border with Tanzania, the park continues in Serengeti; the whole area is famous for the spectacular migration of animals, savanna herbivores such as wildebeest and zebras, moving between the two parks, along a circular path throughout the year, in search of pastures made green by the rains, and are preyed upon by lions, hyenas, leopards and cheetahs, while crocodiles and hippos await them in rivers. In the area we find the Mara River, and its tributaries, while some branches are lost in the swamps.

Wildebeest migration in Masai Mara

In both parks there are two rainy seasons, one that lasts about six weeks and occurs from March to May (called the long rains), and one that lasts about two weeks and occurs from October to December (called the short rains); but between the two adjacent parks, Serengeti, located more to the south, remains quite rainy even in January and February, and becomes dry from June to September, while Mara is quite rainy even from June to September, hence the animals make their way from nearby Serengeti to Mara, arriving usually in July, and then go back in October, when the season of the short rains begins in Serengeti. So in Masai Mara there is not a real dry season, although it has a less rainy season from June to September, with July as the driest month.
Here is the average precipitation.
Average precipitation - Masai Mara
Masai MaraJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)95100145210155956080951001551151405

The animals do not move all together, but in herds of tens of thousands, and their movement can vary from year to year, depending on rainfall and therefore pasture availability; however, in Masai Mara, as we said, the animals are usually visible from July to October, with a higher probability in August and September.
Being the park near the equator, the sun passes the zenith twice a year, in late March and late September, remaining very high above the horizon around noon also in the other months: appropriate protection is therefore necessary, also because we are at high altitudes.
The amount of sunshine in Masai Mara is good all year round, since the sky is often clear, although there may be some afternoon cloudiness, which can give rise to showers and thunderstorms, obviously more frequent in the two rainy seasons.
Masai Mara can be reached by flight, or a car trip of 5-6 hours from Nairobi.

Lions in Masai Mara

When to go

The best time to visit the Masai Mara goes from June to September, being the coolest and (albeit slightly) the driest. However, afternoon showers must be put into account, and a bit of cool (or even cold) at night, while during the day the weather is pleasantly warm.

Alternatively you can go in January and February, the other relatively dry season, when it's warmer, and as we said some showers are still possible; in these months, you can still watch the local fauna, such as big cats.

As we have seen, the period in which the herds of wildebeest and zebras reach the park goes from July to October, with a preference for August and September. October is a bit hotter than the previous months, but on the other hand already in September there is a certain temperature increase, due to the passage of the sun at its zenith; so it can be a little hot by day, and less cool at night. For the rest, the rainfall amount is similar to that of the previous months, until the short rains season begins, which depending on the year can happen in October or November.