The Wildebeest Migration in East Africa, also known as “The Great Migration” takes place between Kenya's Masai Mara and Tanzania's Serengeti National Park and is one of the greatest wildlife spectacles on the planet.
Thousands of wildebeest and zebra’s migrate between the Serengeti and the Masai Mara, constantly driven by their search for fresh grazing. The massive herds are closely followed by predators (lion, hyena and cheetah), making the most of every opportunity to catch their next meal.
The best time to see the migration is during the summer months—generally between June and August—when the wildebeest congregate and prepare to cross the famous Grumeti River.
The mass of grunting wildebeest remain on the Mara grasslands until October or November. Then, as the storm clouds gather in the south, the vast herds return to their breeding grounds which, by the time they arrive, are once again green and lush and the cycle begins again.

Check out this cool interactive chart that lets you see where the wildebeests are during any given month. Click to view.