Biennial:
Occurring every two years. The Legislature meets biennially. NOTE: Biannual means "occurring twice a year."
www.leg.state.or.us/glossary.html
Plant that completes its life cycle in two years and generally flowers in the second year
naturesnaturals.co.uk/Botanicals+Glossary
tercentennial
Something that happens every 25 years is referred to as a "quindecennial." This term is derived from the Latin roots "quinque" meaning five and "decem" meaning ten, collectively signifying a period of 25 years. Quindecennial events can include anniversaries, celebrations, or other recurring occurrences.
"Fortnight" is a term that means a period of two weeks. So, "every fortnight" means something that occurs or happens once every two weeks.
The term for something that happens by itself is "spontaneous."
The term for something that occurs every ten years is "decennial." This term is derived from the Latin word "decennium," which means a period of ten years. Decennial events can include census counts, anniversaries, and various planning cycles.
"Event" is a term that best describes the phrase "something that happens" as it denotes a happening or occurrence at a specific point in time.
An event that happens every other year is called a "biennial" event. This term is derived from the Latin "biennium," meaning "two years." Such events can include things like festivals, conferences, or exhibitions that occur once every two years.
A term for something that occurs every eighteen months is "biennial." However, more specifically, to denote an interval of eighteen months, you might use the term "semiannual" to imply a frequency of twice a year, though it does not precisely match the eighteen-month interval. Another option is "18-month cycle."
bidecadal
pressure
Phenomenon
An event that occurs every thirty years is called a "triennial" event. However, this term can be misleading since "triennial" technically refers to something happening every three years. For events specifically occurring every thirty years, the term "tricenary" can be used, although it is less common.