If you want to know when the Serengeti wildebeest migration happens, the first thing to understand is that it's on going, so there isn’t a quick answer to the question (other than "always"). The trick is working out the where AND the when.  Over the year the animals move from their wet season range on the southern and eastern plains of the Serengeti, to the dry season habitat in the north of the park both north and south of the Mara River.  It involves not only something in the region of 1 million wildebeest (estimates vary considerably year on year) but also an estimated 200,000 zebra and 400,000 gazelles.

While they do their best to look purposeful, the herds don't travel directly, but instead meander in a vaguely clockwise direction moving between the patches of fresh green grass that emerge in response to localised rain showers.   It's difficult to predict exactly where the migration will be in a given month because movements depend so much on weather patterns and these can vary by as much as 3 months, which is why we think mobile camps are such a great idea. Very roughly this is how the migration moves:

December - March: Serengeti Southern Plains and Southern Loliondo for the calving (Feb-March)

April - May: Serengeti Southern Plains to the Central Serengeti for the start of the rut.

June - July: Serengeti Central to Serengeti Western Corridor, where they cross the Grumeti River

August to October: Serengeti Western Corridor to Northern Serengeti, where they encounter the Mara River