Who doesn't love a good math holiday? Most people know about Pi Day (3/14), but there are even rarer days on the calendar like Pythagorean Triple Square Day (9/16/25). However, it's time to bust out the calculators in celebration yet again: November 23 marks the annual Fibonacci Sequence Day.
The poetry of mathematics manifests everywhere in nature, but few numerical patterns are more common than the Fibonacci Sequence. First described in 1202 by mathematician Italian Leonardo Bonacci (Fibonacci is a shortening of filius Bonacci or "son of Bonacci"), the concept involves adding 1 and 1 together, then doing the same for every successive pair of numbers. Each sum will inevitably be the next number in the sequence.
However, amid 13th century Europe, numbers were still most often expressed by using unwieldy Roman numerals. To simplify things, Fibonacci also proposed adopting the Hindu-Arabic numerical system based on the symbols 0 through 9. Although he died sometime between1240 and 1250 in Pisa, Italy, his Arabic numeral suggestion ultimately became the norm across Europe by the 15th century.
The Fibonacci Sequence is a rudimentary idea, but the wider implications are profound. It's also directly related to another concept known as the Golden Ratio that appears in several natural phenomenon including tree growth, the Milky Way galaxy's spiral arms, seashells, and many other venues. Without the Fibonacci Sequence there even wouldn't be a practical way to design today's programming algorithms.
Although technically not an example itself, November 23rd provides the most appropriate day to celebrate the famed thinker for his contributions to math. According to the holiday trackers at National Day, Fibonacci Sequence Day observances can include creating your own Fibonacci spiral or baking a pie with a spiral crust topping. Who says the circular dessert is only reserved for March 14th?