Usually those days are called holidays.
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
It is called Leap Year, and the day that is added is February 29th. We only get that day once every four years.
Something that happens once a century.
A special promotion "sale" that happens once a year.
Something that happens every six years is sexennial.
This is a blue moon. It happens very rarely, once every two to three years or so.
The letter "e".
An event occurring every two years is referred to as a biennial event. This pattern indicates that the event happens once every two years.
The letter 'm'. It appears once in the word "minute," twice in "second," but not at all in "hour."
A millennium happens once every thousand years.
THAT YOUR AGE ON THIS DAY IS2022
The Olympics is an event that happens once every 4 years, bringing together athletes from around the world to compete in various sports.