sexennially

sex·en·ni·al

 (sĕk-sĕn′ē-əl)
adj.
1. Occurring every six years.
2. Relating to or lasting six years.
n.
An event that occurs every six years.

[From Latin sexennium, of six years : sex, six; see sex- + annus, year; see at- in Indo-European roots.]

sex·en′ni·al·ly adv.