according to me it is biennial.
Istanbul Biennial was created in 1987.
sea-weed, protozoa
Singapore Biennial Games was created in 2010.
The word "biennial" can function as both an adjective and a noun.
Carrots, parsley, and onions are some examples.
A biennial plant flowers every two years
The word Biennial means Every two years
The adjective "biennial" means done or occurring every 2 years. (biannual means twice a year)
triennial
It is an adjective.
Adriano González León Biennial Novel Prize was created in 2004.