There is no bad month. There are wrong pairings.
The month you travel matters more than the park you choose. Here is what each part of the year actually looks like.
January and February — hot, dry, excellent
Short dry season. Grass is low, animals concentrate at water, and the light is hard and clear. Calving season in the southern Serengeti, which means predators. One of the two best windows and noticeably quieter than August.
March to May — the long rains
The months operators avoid mentioning. It rains, roads get difficult and some camps close. It is also green, empty, cheap and photographically the best light of the year. If you do not mind a wet afternoon, this is the value window.
June to October — the classic season
Dry, cool mornings, and the migration in the Mara from roughly July. It is also peak: highest rates, camps full a year ahead, and the reserve busy. A conservancy solves most of that.
November and December — the short rains
Brief afternoon storms rather than all-day rain. Green, quiet, good value, and the birding is at its best. December fills up over Christmas, so book early or come in November.
It is a year-round movement, not an event.
The herds are somewhere in the ecosystem every month of the year. The river crossings — the thing people picture — happen roughly July to October, and even then they are unpredictable. Cameras wait days for them.
Come for the migration by all means. Just do not build the whole trip on seeing a crossing, because nobody can promise one.
We will tell you honestly whether they are the right ones.
And where to go given them, which is usually a better question.