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 )
Every 4 years
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.
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.
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.
The winter Olympics are a sporting event Witch occurs every 4 years. They were first held in 1924 in chamonix,france