Yellow Pig Day
Yellow Pig Day is held on July 17. It is a monthly collaborative art project celebrating the United States' wide array of bizarre national holidays. This event in the second decade of the month July is annual. 
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The mascot of the holiday, the yellow pig, has 17 toes, 17 eyelashes, and 17 teeth.
Ah, there are two mathematicians. David C. Kelly and Mike Spivak. They were graduate students together at Princeton in the 1960's. They reportedly got the yellow pig 17 idea at a bar.
Although this might sound like a pork-appreciation day, it's actually a day to enjoy the wonders of mathematics, and particularly the special properties of the number 17. There was even a Yellow Pig Math Days conference in 2006. Apparently one of the things they did at this conference was sing yellow pig carols.
Similar holidays and events, festivals and interesting facts
World Snake Day on July 16
World Emoji Day on July 17 (International is an unofficial holiday. Celebrated annually since 2014, NBC reported that the day was Twitter's top trending item on July 17 in 2015);
Tattoo Day on July 17
Ugly Truck Day on July 20
Get Out of the Doghouse Day on July 20 (The original idea came from the symbolism in the 1911 Peter Pan novel written by J. M. Barrie in chapter16 when Mr. Darling goes out and sleeps in the dog kennel, remorseful of not paying enough attention to his family and allowing his children to be kidnapped);
National Moon Day in USA on July 20
Schwörwoche and Fischerstechen in Ulm on July 20 (A celebration of tradition and cohesion)
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