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)
Well, honey, when something happens 6 times a year, you call it bi-monthly. It's like clockwork, just happening every couple of months like a bad haircut appointment. So, there you have it, bi-monthly - six times a year, like clockwork.
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 term "bi-annual" can be confusing as it can mean either twice a year or once every two years. However, in most contexts, it is commonly understood to refer to events that occur twice a year. If you mean "biennial," that refers to something that happens every two years.
The letter "e".
They interfere.