Tanzania: when, how?
The dry season between June and September is probably the best time to visit Tanzania, when wildlife-watching is generally more rewarding and the weather more reliable.
Assuming this is your first visit, and you’ll be doing the classic northern safari circuit, you’ll find this is the best time for Tarangire National Park – when the park’s large herds of elephant congregate along the Tarangire River and at its waterholes – and for the famous river-crossings in the northern Serengeti, with the migration normally crossing the Grumeti in June and the Mara in July and August. This is also a good time to visit Zanzibar, though note that most restaurants on the island will be closed during Ramadan (around a month from July 9 this year).
That’s not to say you shouldn’t visit at other times of the year. Game-watching is good year-round in the Ngorongoro Crater, and you can just follow the herds around the Serengeti: January–March & December on the Plains and around western Ngorongoro (when you’re likely to see new-born calves); April–June in the centre of the park (when numbers are at their peak); November up in the north and northeast, when they often venture out of the park into Loliondo. A good (but expensive) option for the Serengeti is a mobile tented camp (such as nomad-tanzania.com), which has the flexibility to follow the herds.
Good agencies include:
africatravelresource.com, which is also great for in-depth and impartial accommodation reviews
Hope that helps. Have a great trip!