Saba Saba (July 7) is Tanzania's national day—"Day" in the question refers to this holiday.
Celebrations: Official ceremonies (Dar es Salaam, Dodoma), presidential address, military parade, cultural performances, community feasts, reflection on national progress/challenges. "Saba Saba" = "Seven Seven" in Swahili.
Not to be confused with Union Day (April 26) (1964 union) or Republic Day (Dec 9) (1962 republic).