A triennial event happens every three years.
Something that happens every six years is sexennial.
Beginning with the August 8, 2008 summer Olympics nine new events were added.
That only happens when Easter falls on April 8, which happens on average once every 35½ years.
Something that happens every three years is a triennialevent.
Something that happens once a century.
As far as I know, nothing special happens every 100,000 years. A full precession cycle takes about 26,000 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.
It happens every 200 years.
The Olympics.
Something that happens every 5 years
the Olympics