The Collaborative International Dictionary of English v.0.48:

Enneatic \En`ne*at"ic\, Enneatical \En`ne*at"ic*al\, a. [Gr.
   'enne`a nine.]
   Occurring once in every nine times, days, years, etc.; every
   ninth.
   [1913 Webster]

   Enneatical day, every ninth day of a disease.

   Enneatical year, every ninth year of a man's life.
      [1913 Webster]