Maji Maji Festival in Songea
Songea is the historic center of the Maji Maji Rebellion of 1905-1907, and its Maji Maji Memorial Museum, confirmed as an active heritage site visited even by foreign heads of state, anchors the historical memory a Maji Maji Festival would draw on, though a specific annual festival by that name was not independently verified.
The Maji Maji Rebellion is one of the best-documented events in Tanzanian colonial history: an armed uprising of Africans against German colonial rule in German East Africa fought between 1905 and 1907, with Songea serving as a center of resistance during that period, according to Wikipedia's entries on both the rebellion and the town.