The Collaborative International Dictionary
Octennially
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.
Wiktionary
octennially
adv. Every eight years.