The wildebeest migration in East Africa is an ongoing and cyclical event that doesn't have a fixed start or end date, and it can last for much of the year. The migration is driven by changing environmental factors such as rainfall and the availability of food and water. Here's a general overview of the timeline of the wildebeest migration:

Calving Season (Late January to February): The migration typically begins in the southern Serengeti in Tanzania, where wildebeest herds gather to give birth to their calves. This phase coincides with the start of the short rains and the availability of fresh grass for the young calves.

Dry Season (June to September): As the dry season progresses, the wildebeest herds begin their northward migration, following the rains and the growth of new grass. During this time, they move through the central and northern Serengeti.

River Crossings (July to October): One of the most dramatic phases of the migration is the river crossings, particularly at the Mara River in Kenya. As the herds move north, they encounter rivers that must be crossed to reach better grazing areas.

Return South (November): As the dry season ends and the grasses in the northern areas become depleted, the wildebeest herds start their journey back to the southern Serengeti, often arriving in November. This return migration ensures that they have access to fresh grass for the calving season the following year.

Calving Season Again (Late January to February): The migration cycle begins anew with the calving season in the southern Serengeti.

As you can see, the wildebeest migration is an ongoing and year-round process with different phases. It's a continuous movement of the herds in response to changing environmental conditions. While specific phases of the migration can be witnessed during certain times of the year, the entire migration cycle lasts throughout the year. Travelers who want to witness specific phases of the migration should plan their trips accordingly to coincide with the relevant times of the year.