Fibonacci Day
November 23 · Level 3 · #1708 of 2238
Maths fans honour Fibonacci and his famous number sequence. The date is a neat trick: 23 November is written 11/23 in the American style, and the digits 1, 1, 2, 3 are the start of the sequence itself.
How it is celebrated: teachers walk classes through the sequence, and everyone else hunts for spirals in sunflower heads, pine cones and pineapples, or sketches them on graph paper.
Leonardo of Pisa, nicknamed Fibonacci, introduced Europe to Hindu-Arabic numerals in his 1202 book Liber Abaci. The sequence only got his name in the 19th century, and Indian mathematicians had described it much earlier.