Fibonacci Days occur when your day age lies in the Fibonacci sequence.

In mathematics, the Fibonacci numbers are the numbers in the following integer sequence:

1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...

or (often, in modern usage):

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...

By definition, the first two numbers in the Fibonacci sequence are either 1 and 1, or 0 and 1, depending on the chosen starting point of the sequence, and each subsequent number is the sum of the previous two.

In adult life Fibonacci days become quite rare, so make sure you don't miss any more of them!

Mind you, if you start following them from an early enough age, you can celebrate your 1st and 2nd Fibonacci Days on the same day!