The Collaborative International Dictionary of English v.0.48:

Octennial \Oc*ten"ni*al\, a. [L. octennium a period of eight
   years; octo eight + annus year.]
   Happening every eighth year; also, lasting a period of eight
   years. --Johnson. -- Oc*ten"ni*al*ly, adv.
   [1913 Webster]