The Collaborative International Dictionary of English v.0.48:

Nundinal \Nun"di*nal\, Nundinary \Nun"di*na*ry\, a. [L.
   nundinalis, nundinarius, fr. nundinae the market day, the
   weekly market, prop., the ninth day, fr. nundinus belonging
   to nine days; novem nine + dies day: cf. F. nundinal.]
   Of or pertaining to a fair, or to a market day.
   [1913 Webster]

   Nundinal letter, among the Romans, one of the first eight
      letters of the alphabet, which were repeated successively
      from the first to the last day of the year. One of these
      always expressed the market day, which returned every nine
      days (every eight days by our reckoning).
      [1913 Webster]