Yellow Pig Day originated in the 1960s when two math students from Princeton spent a long time obsessively analysing the number 17. It seems that they went a little mad eventually, and decided to invent the concept of a yellow pig with 17 toes, 17 teeth, and so forth... Now Yellow Pig Day is an important part of the academic calendar, and is celebrated with cake, carols, parades and general revelry.
History

Yellow Pig Day is a mathematician's holiday celebrating yellow pigs (is there such a thing!?!), and the number 17. It is celebrated annually since the early 1960's, primarily on college campuses, and primarily by mathematicians. On campus, Yellow Pig Cake and Yellow Pig Carols are tradition!
If you are a mathematician, spend part of the day thinking and working in multiples of 17. And, while you do so, give a little thought to yellow pigs.
Another Pig Day? There is also some references to a "Pig Day" on January 17th. We found it in online calendars and on Ecard companies. We did not find it in any published documents. We did not find any factual evidence supporting this day on January 17th.
Color of the Day: Yellow, for Yellow Pig Day.
Origin

Ellen Stanley, a Texas art teacher created National Pig Day in 1972. Her intent was to to recognize and be thankful for pigs as intelligent domestic animals.
There is no evidence to suggest that this is truly a "National" day, which requires an act of congress.
The Yellow Pig was the brainstorm of mathematicians Michael Spivak and David C. Kelly in the early 1960's. They were students at Princeton University, studying mathematics. History was made as they were listing interesting properties of the number 17 (can you imagine the interesting properties!?). During this thought wave, the yellow pig was born. Most likely, it had 17 toes, 17 teeth, 17 eyelashes, etc. The yellow pig and the number 17 have been linked ever since.