nothing
A triennial event happens every three years.
Something that happens every six years is sexennial.
It is a triennial event. A triennial event happens three times a year.
bidecadal
Semicentennial
An event occurring every two years is referred to as a biennial event. This pattern indicates that the event happens once every two years.
True.
I think you are asking about a census.
Leap year
Chicago Cubs in the World Series?
An event that happens every other year is called a "biennial" event. This term is derived from the Latin "biennium," meaning "two years." Such events can include things like festivals, conferences, or exhibitions that occur once every two years.
Every 100 years is called a century, made up of 10 decades.