Lewis & Short

trīcennālis, e, adj. [tricennium], of or belonging to thirty years, tricennial (late Lat.): incuria, Ruf. in Hier. 1, 11.
In plur. subst.: trīcennālia, ĭum, n., a festival celebrated once in thirty years, a tricennial festival, Oros. 7, 28 fin.