The Great Wildebeest Migration in Tanzania and Kenya is a year-round event as the herds constantly move in search of fresh grazing and water. However, the timing of their movements can vary depending on the location and time of year. Here's a general breakdown of the migration by month:

December to March: During these months, the herds are typically in the southern Serengeti in Tanzania, often around the Ndutu area. This is the calving season, and you can witness thousands of wildebeest and zebras giving birth to their young. It's a time of plenty for predators, as there are plenty of vulnerable prey.

April to June: As the rains subside, the herds start moving north, transitioning to the central Serengeti. This is when you might see large columns of migrating animals on the move.

June to July: The herds continue their journey northward and typically reach the western corridor of the Serengeti during these months. This is when they prepare for the most dramatic part of their journey: the river crossings.

July to October: This is considered the peak of the migration, with the wildebeest and zebras crossing the Grumeti and Mara Rivers as they head into the Maasai Mara in Kenya. These river crossings are one of the most iconic and dramatic wildlife spectacles in the world.

November to December: As the short rains begin, the herds start their journey back to the southern Serengeti, where they will calve once again, completing the annual cycle.

Please note that the exact timing of the migration can vary from year to year due to factors like weather patterns and the availability of food and water. It's advisable to work with a reputable safari operator and plan your trip based on the specific experiences you want to have during the migration.