vicennial

adjective

vi·​cen·​ni·​al vī-ˈse-nē-əl 

: occurring once every 20 years

Word History

Etymology

Late Latin vicennium period of 20 years, from Latin vicies 20 times + annus year; akin to Latin viginti twenty — more at vigesimal, annual

First Known Use

circa 1859, in the meaning defined above