7. Fibonacci Day (November 23rd)
People celebrate Fibonacci Day in the mathematics community on November 23rd of every year. It is simply because the date, written as 11/23, represents the first four numbers of what we call the Fibonacci sequence. First written about in the book Liber Abaci in the year 1202, authored by, you guessed it, Fibonacci (aka Leonardo Bonacci or Leonardo Bigollo Pisano). A basic explanation of the sequence is this: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 and so on. Each number is the total sum of the two numbers that came before it with the potential to reach infinity.
This sequence of numbers is so standard that it even exists in multiple places in nature. For example, in the leaves of several plants, within a pineapple, or in pinecones of pine trees. Being such a standard mathematical figure, there is even a scientific journal dedicated to it under the name of Fibonacci Quarterly, which is done as a publication of The Fibonacci Association. It publishes four journals a year and has done ever since 1963. The journal publishes related articles, mathematical quizzes, and articles written by invited and distinguished mathematicians.