pentennial
A triennial event happens every three years.
Something that happens every three years is a triennialevent.
Something that happens once a century.
Something that happens every six years is sexennial.
Something that happens every 5 years
Quinquennial = something that happens every 5 years. Lustral = also something that happens every 5 years or we could invent: semi-Decennial: where semi is half and Deccennial happens every 10 years.
tercentennial
Quadrenial
Biennial or biannual can refer to something which happens every two years.
A centennial happens every 100 years on that specific day every 100 years. It's the 100th anniversary of something.
decennial Occurring every ten (10) years. Also tenth anniversary
It is spelled right (biennial) and means something that happens every two years. Example- a flower blooming every two years.