In general, there is no best time to visit the Masai Mara National Reserve and the neighbouring private reserves; but there is, however, a right time for each activity you want to practice.
The Masai Mara National Reserve and the neighbouring Conservancy lie just South of the equator and are characterized by the alternation of two main seasons: the dry season and the wet or green season.
The dry season usually starts in June and ends in October; in these months rainfalls are almost non-existent, the air is dry and the temperatures are not high, the coldest temperatures are recorded between June and July with maximum 25°C and minimum 10°C.
The wet or green season from November to April is characterized by higher temperatures, especially in December and January, with maximum 32°C and minimum 18°C.
In this season there are two rainy periods: the short rains in November and the heavy rains in April and May;  in these months it is difficult to drive on the "black cotton", the typical terrain of this area, that is little permeable and becomes extremely slippery in the rainy season; while from December to March there are no rainfalls at all.
This area is at its best during the wildebeests and zebras migration, which occurs between late June and the end of October; it is worth noting that this calendar is merely indicative because the arrival of these herbivorous is driven by the alternation of seasons and actual weather conditions, and hence is quite unpredictable when they move to these plains and return to the Serengeti National Park in Tanzania.
As a general rule, in the dry season it is easier to spot the wild animals who gather near water courses, that are vital for their survival, and vegetation is low; while in the wet season the vegetation is lush, so sightings are more difficult and the animals are more dispersed.
For birdwatchers the best months are from November to April because in this period, in addition to the resident species, migratory birds, coming here from Europe to spend the winter, are also present; November and April are the wettest months and are not the best for visiting the area although rainfalls consist of quick afternoon showers.
January and February are the best time to witness the birth of wildebeest, zebra and antelope puppies and experience the amazing view of chasing scenes by predators who seek to take advantage of the vulnerable situation of the newborns and their mothers.