septennial
adjective
sep·ten·ni·al sep-ˈte-nē-əl 
1
: occurring or being done every seven years
2
: consisting of or lasting for seven years
septennially
sep-ˈte-nē-ə-lē
adverb 
Example Sentences
Recent Examples on the Web The former English schoolkids, tracked by this septennial doc since age 7, are now using the medium to promote their interests. 
Wired Staff, WIRED, 1 Oct. 2006
Wired Staff, WIRED, 1 Oct. 2006 Word History
Etymology
Late Latin septennium period of seven years, from Latin septem + -ennium (as in biennium)
First Known Use
1640, in the meaning defined at sense 1