November 23·Every year

Fibonacci Day

November 23rd — 11/23 — spells out the start of the Fibonacci sequence: 1, 1, 2, 3. Leonardo Fibonacci introduced the sequence to the Western world in 1202, though Indian mathematicians had described it 1,300 years earlier. The sequence appears in the spirals of sunflowers, the branching of trees, the arrangement of pine cone scales, and the nautilus shell — nature's preferred growth algorithm.