Triennial
Triennial, from Latin "triennium".
Every decade
The word occurring has three syllables. (Oc-curr-ing)
The word is... bicentennial.
Consecutive.
Biennial means "occurring every two years," OR "lasting for two years." The word biannual can also mean "occurring every two years," although its most common meaning is "occurring twice a year."
enneatical
Biannual. Biannual can mean occurring every two years, OR occurring twice a year. Biennial, on the other hand, means only the former: occurring every two years.
Triennial, from Latin "triennium".
Centenary.
"bi" is prefix meaning every other, but it can be any time period. For example: Biweekly = every other week bimonthly = every other month Biennially = every other year
Every day is a phrase meaning every day. It takes its stress on the "day." Everyday is a word meaning "commonplace." It takes its stress on the "ev-".
Bientum. Coming for the Latin word "Centum" meaning one hundred.
annual or anniversary
The word for every 75 years is "septuagesimal."
Biennial is a word describing an event that happens every two years.
The word for every twenty years is "score."