Yellow Pig Day

Yellow Pigs Day or Yellow Pig Day (July 17) is a holiday celebrated by some mathematicians, in recognition of the Yellow Pig and the number 17.

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.
Michael Spivak has since written many famous math textbooks which contain yellow pigs. Spivak's references to yellow pigs are not always direct; some textbooks contain references to Chinese policemen. David Kelly runs Hampshire College Summer Studies in Mathematics (HCSSiM), a summer program for mathematically-talented high school students, all of whom have been introduced to the 'cult' of the Yellow Pig.
References to yellow pigs are not uncommon among mathematicians, particularly those who attended HCSSiM or worked with Spivak or Kelly. Several mathematicians maintain collections of yellow pigs.

Yellow Pigs Day events 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 most elaborate festivities are held at HCSSiM, where the holiday has been celebrated annually for roughly 34 years. In preparation for the event, 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.
David Kelly gives a lecture on the historical and mathematical properties of the number 17, and his impressive collection of well over 289 (17 squared) yellow pigs is on display. The celebration concludes with the singing of Yellow Pigs Day carols and the eating of a yellow pig cake. Gifts containing or consisting of yellow pigs and mathematical knick knacks featuring the manifold properties of 17 are frequently exchanged among friends.