Lewis Short
(adj.adv.) : trĭnundĭnus, a, um, adj.tres nundinae
* Of or belonging to three weekly market-days (very rare): trinundino die,after seventeen days,Macr. S. 1, 16, 34; v. trinus, II. B., and nundinus, III. — Hence, adv.: trĭnundĭnō, = trinundino die, Macr. S. 2, 13, 7.
Charlton T. Lewis, Charles Short, A Latin Dictionary