Yellow Pigs Day Date in the current year: July 17, 2026

The Hampshire College Summer Studies in Mathematics (HCSSiM) is a residential program for mathematically gifted high school students that originated at Hampshire College in Amherst, Massachusetts. David C. Kelly, one of the college’s mathematics professors, founded it in 1971, and its association with yellow pigs also comes from Kelly.

Kelly had a lifelong fascination with the prime number 17 and its unique properties. While studying at Princeton in the early 1960s, Kelly and a fellow student, Michael Spivak, analyzed the number 17 for an assignment and came up with the idea of a fictional yellow pig with 17 toes, 17 teeth, and 17 eyelashes. Later, they created an annual celebration of the number 17 called Yellow Pigs Day. Although Kelly refused to divulge the exact origin of the yellow pig and Yellow Pigs Day, this is the most commonly cited origin story of the celebration.

When Kelly became involved with HCSSiM, the yellow pig became the program’s mascot, becoming so closely associated with HCSSiM that the program is now administered by the YP Math Foundation, YP standing for “yellow pig”. And Yellow Pigs Day became a day when many alumni returned to campus for a traditional celebration with cake and Yellow Pig carols from the HCSSiM Yellow Pig Songbook. Many sources call it Yellow Pig Day, but the HCSSiM insists that it’s “pigs”, plural, which is fitting given Kelly’s extensive collections of yellow pigs.

Outside of HCSSiM, math enthusiasts fascinated with mathematics in general and the number 17 in particular have adopted the celebration of Yellow Pigs Day, as have people who love quirky holidays. This is a perfect opportunity for non-mathematicians to learn some interesting facts about the number 17 from a lecture that Kelly used to give each summer.

17 is a prime number, meaning it can be evenly divided by only two numbers: 1 and itself. Due to psychology experiments in which people were asked to choose a random number between 1 and 20 and many participants independently chose 17, it has been referred to as the “least random number”.

However, Italians consider 17 to be unlucky, similarly to how many other cultures consider 13 to be unlucky. They believe Friday the 17th is an unlucky day, many high-rise buildings in Italy lack a 17th floor, and some Italian airlines omit row 17 from their planes. One reason for this superstition, called heptadecaphobia, is that the Roman numeral for 17 (XVII) is an anagram of the Latin word vixi, meaning “I am dead”.

Many fascinating events occurred on Yellow Pigs Day long before the day itself existed. For example, Handel’s Water Music premiered on this day in 1717, the Apollo-Soyuz “handshake in space” took place on this day in 1975, and seven polar bears escaped from their moated grotto in the Brookfield Zoo in Chicago and pillaged a concession stand on July 17, 1969.