July 17 is designated as Yellow Pig Day. 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.
Mr. Spivak has since written many famous math textbooks which contain yellow pigs. His references to yellow pigs are not always direct... some are Easter Eggs and some textbooks contain references to Chinese policemen. Mr. Kelly runs Hampshire College Summer Studies in Mathematics, 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 have worked with Spivak or Kelly or know of their work. Several mathematicians maintain large collections of yellow pigs.