Annual or annually.
Once every year.
Each/every/once a year.
Biannual describes something that occurs twice a year, once every half year.
A similar word for "annually" is "yearly." Both terms refer to something that occurs once every year. Other synonyms include "once a year" and "per annum."
The word for an event that happens every year is "annual." This term is commonly used to describe recurring events, such as festivals, meetings, or celebrations that take place once each year.
once every year
Usually those days are called holidays.
The letter 'M'.
Once.
once every year
The letter E (once in a minute, twice in a week, and once in a year)
The letter "E" occurs four times in the word "week," twice in the word "month," and once in the word "year." This riddle plays with the frequency of the letter rather than any specific event or occurrence.