List of fictional holidays

This is an incomplete list.

Crimboween - Combination of Christmas and Halowe'en - Kingdom of Loathing
Do As We Say day - created by German settlers in 1945 - The Simpsons
Do What You Feel Day - Self-explanitory - The Simpsons episode Bart's Inner Child.
Durin's Day - Dwarven equivalent of New Years Day. The secret entrance to the Mountain above Dale and Esgaroth becomes visible. - The Hobbit
Eliza Doolittle day
Great Swap Meet - Jawa equivalent to Christmas - Star Wars
Gurnenthar's Ascendance - referenced in the Buffy the Vampire Slayer episode "Into the Woods"
Hogswatch in Discworld
Opposite Day - A day when everything is backwards. - Various works of fiction.
Pizza Day - Dwarven holiday. Several thousand Dwarves head to the very first pizza parlour, Stonehenge. It is much like Christmas, as it has at least one song related to it. - Artemis Fowl
Weasel Stomping Day - Day when every one spreads mayo on the lawn, puts on viking helmets, and boots, then proceeds to kill weasels by stepping on them - Weird Al Yankovic song of the same name
Whacking Day - The day where every body beats snakes to death with clubs - The Simpsons
Yule - The Hobbit equivalent to New Year's Eve and New Year's Day - The Lord of the Rings
First Contact Day - celebrated on April 5th in the Star Trek universe(s). On this day, humans achieved WarP drive-powered space flight for the first time, and later that day, met for the first time with alien beings - explorers from the planet Vulcan.
Hogswatch in Wincanton Somerset England Roundworld.

Celebrated every year in December since 2002. This year held on 1-2 December, attended by Terry Pratchett and one or two other discworld notables.

Decemberween - Fictionalized version of Christmas, celebrated on December 25 - Homestar Runner
Festivus - December 23. Celebrated with the Airing of Grievances and the Feats of Strength. Traditional decoration is an unadorned aluminum pole. Popularized in Seinfeld, originally invented by Daniel O'Keefe