Couples participating in the cheesy celebrations were selected by &pizza after applying in advance. The weddings — complete with a ceremony, flowers, photography and plenty of pizza — are free to the couples. Pi, for those who need a geometry refresher, is the ratio of a circle’s radius to its diameter. It’s a mathematical constant rounded to 3.14, so, naturally, Pi Day falls on March 14. It’s a “holiday” often celebrated by pizzerias and pie purveyors alike.