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
An event occurring every two years is referred to as a biennial event. This pattern indicates that the event happens once every two years.
I think you are asking about a census.
Chicago Cubs in the World Series?
Leap year
True.