Most safari guides will tell you to go between July and October — that is when the Great Migration crosses the Mara River. They are not wrong, but the answer is more interesting than that.
The Mara has two rainy seasons. The long rains (April – May) and the short rains (November). Outside of those windows the reserve is open and beautiful, and the green season produces some of our favorite light of the year.
For first-time visitors we usually recommend late July through early September. For honeymooners and photographers we love February — fewer vehicles, dramatic skies, and predator activity peaks as the herds move south.