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 occurs once every 100 years is known as centennial.
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.
Quadrenial
tercentennial
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