Clear skies and beautiful days. Amazing river crossings and excellent resident game. One of the very best times to be in the Serengeti – better than August, in my opinion, due to a quieter park.
In September, the herds are pretty much in an identical position to where they are in August, in the Lamai wedge and Kogatende areas of the northern Serengeti, as well as sometimes up in the Masai Mara too. Many people think that the Masai Mara is the place to be at this time, but that is simply not true, as the wildebeest tend to pour out into Kenya as an extension of their route, and as such, entering the Mara is not an integral part of their journey.
So all in all, you should be looking at the Serengeti over the Mara for the river crossings.