Yellow Pig Day is celebrated on July 17, 2018. It have been held by mathematicians since at least the 1960s. Most celebrations occur on July 17, although others are held on May 17 (May being the "17th month" of the previous year) and September 17 (a convenient time for college students).
The Yellow Pig is believed to have originated with mathematicians Michael Spivak and David C. Kelly, while they were students at Princeton University in the early 1960s. They began listing interesting properties of the number 17, and somehow the 17-eyelashed yellow pig was born. References to yellow pigs are not uncommon among mathematicians. Several mathematicians maintain collections of yellow pigs.
In preparation for Yellow Pig Day, students decorate shirts (often mathematically themed) with yellow pigs on them. On Yellow Pigs Day students, staff, and visiting alumni wear these shirts. Ultimate has long been the official sport of the day, and a competitive student/alumni game is held. In recent years, origami yellow pigs have been folded. (With material from: Wikipedia)