The migration is a loop, not a festival with a start time. Wildebeest spend roughly July to October in the Mara, cross back toward the Serengeti as the short rains build, and calve on the southern plains around February. If your only week is the first of August, book it. If you can choose, read the grass, not the poster.
River crossings are the photograph everyone asks for, and they are the least reliable hour of the trip. Herds pile on a bank for days and then cross in twenty minutes while you are at lunch. A good guide waits. A bad itinerary has you on the road to the next lodge.
Outside those months the Mara does not close. Resident lion prides, cheetah on the short-grass plains, and elephant in the marshes are here in January. You will share the reserve with fewer vehicles. We say that plainly to guests who feel they have “missed” Africa because they cannot travel in August.
What we actually do: hold a conservancy or reserve camp that can position you at dawn, keep the vehicle private if the budget allows, and refuse to promise a crossing on a Tuesday. If the herd is south of the Sand River, we will tell you before you fly.