Octennial

Oc·ten'ni·al adjective [ Latin 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 , adverb
Found on http://www.encyclo.co.uk/webster/O/9

Octennial

• (a.) Happening every eighth year; also, lasting a period of eight years.
Found on http://thinkexist.com/dictionary/meaning/octennial/
No exact match found