Pi is a numerical ratio, which relates the circumference of a circle to its diameter.

The Pi Day is celebrated on March 14 (3/14 in American notation), with 3.14 being the best known approach to this mathematical constant.

In 2012, by the initiative of the Portuguese Mathematical Society, CTT - Correios de Portugal issued a postal stationery dedicated to that date. The picture-postcard of the event, shows the number pi 3.14159265 ... in the left side, and the respective Greek letter on the stamp with the date 14.03.2012