When is migration time in the Serengeti?

Unfortunately there are no guarantees when it comes to the timing of the migration. Much is dependant on the rain and the seasons start at different times every year which means it is difficult to predict their exact migration at any given time.
Generally in January, large herds of wildebeest, zebra and gazelle can be seen on the plains of the southern Serengeti.
February: The wildebeest begin foaling. This naturally attracts the predators.
March-April: Sees the start of the rainy season and the herds start migrating northwards along the western corridor of the Serengeti.
May-June: The herd moves towards the Grumeti where there is still good grazing.  They also begin their crossing of the crocodile infested Grumeti river which provides for the stuff wildlife documentaries are made of...
July-August: The herd is moves further north towards the Masai Mara in search of better grazing. They also have to cross the dangerous Mara River to reach the grazing plains of Kenya.
Heavy rains November 2007 have really confused the migratory herds! While some herds still remain in the Masai Mara, others were scattered in the Lobo region, while another large herd made its way via the Grumeti region to the Nyamuma plains.
It is advisable to keep updatd on their movement...

Just a interesting fact, the name Tanzania comes from Tanganyika + Zanzibar = Tanzania