Usually those days are called holidays.
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
Something that happens every 100 years is called a centennial. It marks a significant milestone or anniversary reaching the 100-year mark.
The letter n
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.
Birthdays.
The letter "e".
This is a blue moon. It happens very rarely, once every two to three years or so.
A millennium happens once every thousand years.
THAT YOUR AGE ON THIS DAY IS2022