Triangular Numbers Day - October 15
It's Triangular Numbers Day, the day we all joyously celebrate the triangular numbers ! The triangular numbers are the number of figures needed to create an equilateral triangle. They are also the numbers representing the sum of each of the natural numbers : 0 0+1 = 1 0+1+2 = 3 0+1+2+3 = 6 0+1+2+3+4 = 10 etc. This summation is probably the most useful characteristic of triangular numbers; after that they're probably more of a curiosity. A list of the start of the series can be found here . October 15 was chosen as Triangular Numbers Day because 10/15 is the latest date that can be formed using consecutive triangular numbers.