November 23 is celebrated as Fibonacci day because when the date is written in the mm/dd format (11/23), the digits in the date form a Fibonacci sequence: 1,1,2,3. A Fibonacci sequence is a series of numbers where a number is the sum of the two numbers before it. For example: 1, 1, 2, 3...is a Fibonacci sequence. Here, 2 is the sum of the two numbers before it (1+1). Similarly, 3 is the sum of the two numbers before it (1+2).
This introduction was borrowed from this awesome website. If you're looking for more events for this great Math Holiday, check out their page and some of their great suggestions.
Have a happy Fibonacci Day and remember, everyday can be a great day to celebrate math!