Sukuma Cultural Festival in Mwanza
The Sukuma are confirmed as the largest ethnic group in Tanzania, based in the Mwanza area, and the real Sukuma Museum at Bujora, near Mwanza, is a confirmed active cultural institution documenting their heritage, though no independently confirmed Sukuma Cultural Festival by this exact name and date could be located.
Wikipedia confirms the Sukuma, Swahili Wasukuma, as a Bantu ethnic group from the southeastern African Great Lakes region and explicitly the largest ethnic group in Tanzania, a striking fact given Tanzania's more than 100 documented ethnic groups.