sep·ten·ni·al 
(sĕp-tĕnē-əl)
(sĕp-tĕnē-əl)adj.
1. Occurring every seven years.
2. Consisting of or continuing for seven years.
n.
An event that occurs every seven years.
[From Late Latin septennium, period of seven years, from Latin septennis, of seven years : septem, seven; see sept in the Appendix of Indo-European roots + annus, year; see at- in the Appendix of Indo-European roots.]
sep·tenni·al·ly adv.