Tanzania's calendar is really the Serengeti's migration calendar with everything else layered around it. December through March, the herds calve on the short-grass plains of the southern Serengeti and Ndutu — extraordinary predator action, dry and pleasant weather, and far fewer travelers than the Mara sees in July. June through July, the migration pushes north through the central and western Serengeti corridors. August through October, it crosses into Kenya's Mara ecosystem, though the northern Serengeti's own Mara River crossings run through this window too and are calmer than their Kenyan counterpart. April and May are Tanzania's true low season — heavy rain, some camps closed, migration dispersed and hard to predict — but Ngorongoro Crater's resident wildlife is unaffected by any of this and remains excellent year-round regardless of season, since its animals do not migrate. Zanzibar runs on its own separate calendar (see our Zanzibar and Seychelles timing guide), which is exactly why a June-through-October safari-plus-beach combination works so cleanly: you are in Tanzania for the best mainland season and the coast is simultaneously at its driest.