Pi

The first known use of π on its own for the circle constant, circumference divided by diameter, is in Synopsis Palmariorum Matheseos, a 1706 book by William Jones, a mathematician from Wales, where the letter stands beside the word “Periphery”. Leonhard Euler adopted it, and his books of 1736 and 1748 spread it across Europe.

The digits begin 3.14159 and continue forever without repeating. Americans write dates month first, so 14 March reads 3/14, which makes it Pi Day.