Halloween
Halloween, also known as All Hallows' Eve, Allhalloween and Hallows' Evening, was a holiday celebrated by nightly partying and costuming, which had something to do with spookiness.
Nature
According to one acount, “in some form, in some shape, in some variation, almost every inhabited universe [had] its own version of Halloween” — not necessarily under that name, but recognisable as an autumnal holiday celebrating “the general spookiness of things”. This led some of “the greatest dimensional scholars” to propose possible causal reasons for this state of affairs, with the Hidden Academics of the Tower of Yqwsl speculating that “some unwary dimensional traveler accidentally warped back to the creation of the Multiverse itself (an event which is, in itself, hotly debated) and dropped a pumpkin, or perhaps a ghost, thus sparking the whole thing”. Lord Thymon apparently told the Crew of the Copper-Colored Cupids' Department of Festivities that he once “threw an enormous fright-themed party on the very concept of October 31, thus spreading said party, in various incarnations, across every occurrence of this date in the Multiversal timeline”.
At any rate, in many universes, including the Prime Universe, the walls between the mortal realm and other, more supernatural realities such as the Spirit Realm were weakened on Halloween, with various spirit paths opening. However, spirits who escaped their Spirit Realm on Halloween could not linger past the end of the day in any place to which they were not “anchored”. Similarly, in the Neazzarack Realm, one particular entity, the Spirit King Wrythk, escaped from the Underworld every Halloween, which, as a result, was not a holiday but a time of great strife for the inhabitants of the mortal plane. (PROSE: The Ghosts and the Machine)
Every year, Strach, the Abstract of Terror, organised a Halloween party attended exclusively by ghosts, demons and other spirits at D'Ismay Mansion. (COMIC: Jenny Everywhere in the House of Terror)
History
In 2006, the Jenny Everywhere of the Infinity Apartment celebrated Halloween with her friends and companions. For the occasion, she dressed up as Officer Jenny. (COMIC: Infinity Apartment)
By Halloween 2019, it had become tradition for the Crew of the Copper-Colored Cupids to use Frankenstein-818's Spirit Realm Gateway, which only worked on Halloween, to travel into the Prime Universe's Spirit Realm in Fog Ships and romanticise spirits. However, for unknown reasons, the Gateway malfunctioned. When Frankenstein tried to “fix” it by removing a bunch of parts, it backfired into a fully-fledged Rift which allowed the Great Ghost's legion to escape into the Homeworld. From there, they stole a fleet of Ships and also travelled to the Interdimensional Tavern, the Workshop of Madame Tarsa and the Prime Universe, gathering a supply of host bodies that would allow them to linger in the mortal realm beyond Halloween. However, they were ultimately defeated by the Cupids and Madame Tarsa, allowing the Cupids and the Tavern to resume their respective celebrations. Tarsa, who had been working on a wooden, endlessly re-carvable pumpkin as this year's Halloween novelty but been side-tracked by the incident, instead decided to market clockwork spirits, into which the real spirits had been transformed by the Reverse Spirit Realm Gateway, as toys. (PROSE: The Ghosts and the Machine)
In 2021, Jenny Everywhere invited herself to Strach's party to give Strach a message on behalf of his cousin Lit; for the occasion, she used her shifting to give herself a ghost costume, also noting that she could “do a mean werewolf”. (COMIC: Jenny Everywhere in the House of Terror)