Fibonacci Day
Next: November 23, 2026
One, one, two, three, and onwards.
The story
Where it comes from
Fibonacci Day is an informal day with no known founder or organising body. It falls on 23 November because, written month first as 11/23, the date reads 1, 1, 2, 3: the start of the Fibonacci sequence, in which each number is the sum of the two before it. The sequence is named after Leonardo of Pisa, known as Fibonacci, whose Liber abaci of 1202 posed a puzzle about breeding rabbits that produces it. The same book helped bring Hindu-Arabic numerals to Europe.
How it's celebrated
Maths teachers use the day for puzzles, and people share the sequence and its spirals online. Easy ways to join in: continue the sequence as far as you can (5, 8, 13, 21 …), try Fibonacci's rabbit puzzle, or count the spirals on a pine cone or sunflower head, which usually come in neighbouring Fibonacci numbers. In countries that write the day first, such as Malaysia and Singapore, the date takes a moment of explaining.
Did you know
- Fibonacci learned what he called 'the art of the Indians' nine symbols' as a boy in Bugia, now Béjaïa in Algeria, where his father represented Pisan merchants.
- In India, the Jain scholar Hemachandra worked out the same rule around 1150, while counting rhythms of short and long syllables in poetry, about fifty years before Liber abaci.
- A scholarly journal, The Fibonacci Quarterly, is devoted entirely to mathematics related to the sequence.
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- International Day of Engagement in Science for Sustainable Development · November 27, 2026