Where does “decennially” come from?
decennially (English) comes from English decennial, from Latin decennialis, from Latin -ennium, from Latin -ennis, from Latin annus, from Proto-Italic atnos, from Proto-Indo-European h₂et-no-, from Proto-Indo-European h₂et- — grass.
decennially (English): Every ten years
Ancestry of “decennially”, step by step