An annual event starts with A and happens once a year.
Something that happens once a year.
An annual event
School starts. Fall starts. The Jewish New Year (Rosh Hashanah) starts.
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
a leap year
bicentennial means when something happens twice in a year, or once every two years
The Olympics is an event that happens once every 4 years, bringing together athletes from around the world to compete in various sports.
If something is biannual, it happens twice a year.
Australia Day
"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".
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