octennial
/ɒkˈtɛnɪəl
/adjective
1Recurring every eight years.But this explanation appears to be inconsistent with the octennial period of the festival.
1.1Lasting for or relating to a period of eight years.The Bill was, it is true, changed from a septennial to an octennial one.
Origin
Mid 17th century: from late Latin octennium 'period of eight years' + -al.