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A triennial event happens every three years.
weekly
Georgia winning a national championship in football.
Annual.
A millenium.:)
I think you are asking about a census.
an idiotical freak
Normal, typical, usual, ordinary, common, commonplace, routine, customary, and a daily occurrence.
An annual event
triannual
Month
When you foresee something and it happens, some people call it dejávu, others may call it prophecy. Depends on your forté