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.
The Olympics is an event that happens once every 4 years, bringing together athletes from around the world to compete in various sports.
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
A leap year happens only once every four years; that is also true of US Presidential elections.
bicentennial means when something happens twice in a year, or once every two years
a leap year
If something is biannual, it happens twice a year.
Australia Day
Birthdays.
"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".