Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.