December 31 is the 365th (366th on leap years) and last day of the year in the Gregorian calendar. The next day is the first day of the year in the Gregorian calendar.
- 1944: Neil Ross the voice of the Cyclops was born.
- 1999: "SB-129" and "Karate Choppers" premiered.
- 2002: Earliest possible date of Chocolate with Nuts
- 2009: "Banned in Bikini Bottom" and "Stanley S. SquarePants" premiered in Poland.
- 2015: "Grandpappy the Pirate" premiered in Wales (UK).
- 2017: "No Pictures Please" and "Stuck on the Roof" premiered in Israel.
- 2018: "Library Cards" premiered in Germany, Austria, and Switzerland.
- December 31 on Wikipedia
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