If you do nothing else, visit the Serengeti during one of the semi-annual migrations. More than a million wildebeest, 200,000 zebra and various predators take part in one of the largest animal migrations on the planet. Typically, the migration north takes place between June and August. Aim to arrive just before the wildebeest cross the Grumeti River, and you are certain to see a lot of animal interaction. The return journey arrives in the Serengeti roughly around the end of January. As the migration doesn't take place on a specific date, ask about the cost of changing flights when you make reservations.
Avoid the high temperatures by arriving in Tanzania sometime between July and November. July through September yields the least amount of rain, which means grasses recede and viewing opportunities increase. The short rains occur October to November, but this is still a good time to visit.
During the peak tourist season of July and August, safari companies everywhere will be jockeying for the best position to see the landscape from their Range Rovers. If this is the only time you can go, head to central Tanzania and Ruaha National Park, which is almost always empty (and has large packs of African wild dog---a rare sight).
Tour prices are cheaper during the rainy season, which is April to June. Although some days heavy rains fall for a couple of hours, safaris depart frequently to the less remote parks. Fewer visitors mean you will not have to wait behind a line of SUVs to see the action. Animal concentrations in the southern part of the Serengeti peak at this time, and the grasses are a lush green.
