You are looking for the words semi-annual, bi-annual, twice a year or seasonally.
quarterly
Something which happens every second year is termed biennial
bicentennial means when something happens twice in a year, or once every two 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.
The adjective "biennial" means done or occurring every 2 years. (biannual means twice a year)
When something happens 6 times per year, it is referred to as occurring "bi-monthly." This means the event or frequency happens every two months. It is important to note that the term "bi-monthly" can sometimes be ambiguous, as it can also mean twice a month in some contexts. To avoid confusion, it is best to specify whether it is every two months or twice a month.
You grow an extra limb or two.
The BI word that means twice every year is "biannual." It refers to events or occurrences that happen two times within a year, typically spaced six months apart. It's important not to confuse it with "biennial," which refers to something that happens every two years.
"Once a year" is an adverb (like "sometimes" or "often") and therefore can't be made plural. It stay the same even when describing multiple actions, people, or events. (for example, "Christmas and Easter both happen once a year," or "John and I go to the mountains once a year." If you would like to describe something that happens more often, try "twice a year", "three times a year", "ten times a year", "many times a year", or "once a month". For something less often try "once every two years" or "once every three years".
The letter "e".
They interfere.
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