From June the rain stops, the grass dies back and the land dries out. Animals are forced to gather at permanent rivers and waterholes, and because the vegetation has thinned they are far easier to see. In Tarangire the elephant herds along the river in September and October are genuinely astonishing.
Roads are firm, tracks are passable everywhere, there are almost no mosquitoes, and the skies are clear. Days are warm and dry, mornings and evenings are cold — the Ngorongoro rim can drop close to freezing before dawn, which surprises people.
This is peak season, and everything follows from that: the parks are busier, the best camps sell out six to twelve months in advance, and rates are at their highest. July, August and the Christmas period are the tightest weeks of the year. If those dates are fixed for you, book early rather than shopping around.