nothing
A triennial event happens every three years.
It is a triennial event. A triennial event happens three times a year.
Something that happens every six years is sexennial.
bidecadal
Semicentennial
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.
the word is novennial....
The word "biennial" means an event that either lasts for two years, or it happens every two years.