the word is novennial....
Something that happens every six years is sexennial.
Semicentennial
A triennial event happens every three years.
It is a triennial event. A triennial event happens three times a year.
Biennial or biannual can refer to something which happens every two years.
Something that happens every six years is sexennial.
Semicentennial
The word "biennial" means an event that either lasts for two years, or it happens every two years.
A triennial event happens every three years.
It is a triennial event. A triennial event happens three times a year.
nothing
bidecadal
I think you are asking about a census.
Leap year
Chicago Cubs in the World Series?
True.
Every 100 years is called a century, made up of 10 decades.