The first thing to unlearn: the Great Migration isn't a date in a diary. Around two million wildebeest, zebra and gazelle move around the Serengeti-Mara ecosystem continuously, all year, following rain and grass. There is no month when it isn't happening and no month when it's everywhere.
The circuit, roughly
December to March, the southern plains. The herds spread across the short-grass plains of the southern Serengeti and the Ndutu area. Calving peaks around late January and February, when something like half a million calves are born in a few weeks. Enormous herds on open ground with every predator in the ecosystem nearby. For many people this is the best time of the whole year and it's much less talked about than the crossings.
April and May, moving north-west. The long rains. The herds begin drifting towards the western corridor. Green, quiet, cheap, and some roads are hard going.
Late May to July, the western corridor and the Grumeti. The columns form and the first river crossings happen at the Grumeti.
July to October, the north and the Mara River. The famous crossings, in the northern Serengeti, back and forth across the Mara River, often several times. See the crossings article.
November, turning south again. The short rains pull the herds back down through the eastern Serengeti towards the plains, and it starts over.
Why nobody can give you a date
Because the herds follow rain, and rain moves. A dry patch can hold them north for three weeks; an early storm can pull them south a month ahead of last year. Operators who promise you'll see the crossing in the third week of August are promising weather. What a good itinerary does instead is put you in the right part of the Serengeti for your month and give you enough nights there to be lucky.
What actually makes the difference
Nights, and position. Two nights in the right area is a gamble, four is a reasonable bet. A camp an hour from the action costs you the early morning, which is when everything happens. Mobile camps that move with the herds through the year exist for exactly this reason.
And the part nobody mentions
Even without a crossing, standing beside a herd that stretches past the horizon in every direction, with the noise it makes, is the thing people actually describe when they get home. Our month-by-month view is on the migration calendar.
It isn't only wildebeest
Several hundred thousand zebra travel with the herds, and they're not passengers. Zebra eat the tough upper grass and the wildebeest follow for the shorter growth underneath, so the two species feed each other's grazing. Zebra also see better and wildebeest hear better, which makes the mixed herd harder to ambush than either would be alone. Thomson's gazelle come along behind for the shortest grass of all.