December 1st: Basketball Day
December 2nd: Mutt Day
December 3rd: International Baboon Day
December 4th: Cookie Day
December 5th: Bartender Appreciation Day
December 6th: Cotton Candy Day
December 7th: Toilet Paper Appreciation Day
December 8th: World Techno Day
December 9th: Dewey Decimal System Day
December 10th: Human Rights Day
December 11th: Kaleidoscope Day
December 12th: Cocoa Day
December 13th: World Choral Day
December 14th: Yoga Day
December 15th: Cupcake Day
December 16th: Chocolate-covered Anything Day
December 17th: Christmas Carol Day
December 18th: Underdog Day
December 19th: National Oatmeal Muffin Day
December 20th: Games Day
December 21st: Crossword Puzzle Day
December 22nd: National day of nut bread
December 23rd: Festivus
December 24th: Eggnog Day
December 25th: Ugly Christmas Sweater Day
December 26th: Candy Cane Day
December 27th: Visit the Zoo Day
December 28th: Chocolate Day
December 29th: YMCA Day
December 30th: Bacon Day
December 31st: Make Up Your Mind Day

Cites:
https://mikekerr.com/fun-wacky-silly-offbeat-december-theme-days-and-holidays

December is unofficially the shortest month of the year