Biennial (biannual is twice a yesr)
An event that occurs once every three years.
a persons birthday on the day of a leap year
People born on a double leap year
El Nino.
Eventual world climate change
Eventual world climate change
When used as an adjective, it means occurring every, or last for, four years. As a noun, it references an event that occurs every four years. (see: http://dictionary.reference.com/browse/quadrennially )
Biannual can be used in a sentence as an adjective to denote an event that occurs twice a year, or every two years. So, "It's a biannual event" is a rather ambiguous statement.
Every 4 years
An example of an occasional climate event that occurs every 3-8 years is the El Niño Southern Oscillation (ENSO). It is a climate pattern that involves the periodic warming of sea surface temperatures in the central and eastern tropical Pacific Ocean, leading to significant impacts on weather patterns worldwide.
An event that occurs every 18 months is often referred to as a "biennial" event, although "biennial" typically means every two years. In a more specific context, it may be described as an "18-month cycle" or "semiannual" depending on the nature of the event. In scientific or astronomical contexts, specific terms might be used to denote cycles of that length, but there isn't a widely recognized single term for an 18-month event.
The 29th of February. It only occurs every 4 years and it is at least 12 years and can be as much as 40 years before it occurs on the same day again.