Halloween

Halloween, alternatively spelled Hallowe'en, was a human holiday traditionally celebrated on 31 October. It was customary to "dress up" on this occasion. (COMIC: Imaginary Enemies, TV: The Woman Who Fell to Earth, PROSE: The Golden Hendecahedron) Candy was also associated with Halloween. (TV: Rendition)

History
Harry Houdini died in Detroit on Halloween 1926. (PROSE: Who-Dini?)

On Halloween 1938, Orson Welles broadcast a radio play adaptation of H. G. Wells' novel The War of the Worlds, convincing certain members of the American population that Mars had really invaded. (AUDIO: Invaders from Mars)

At some point during the 1990s, Amelia Pond made Rory Williams dress up as the "raggedy Doctor" for Halloween. (COMIC: Imaginary Enemies)

One Halloween in the 2000s, sometime after Tom stopped travelling with Iris Wildthyme and began living with Panda, Tom dressed up in drag and called himself "Eartha Wydenfire". (PROSE: The Golden Hendecahedron)

On Halloween 2008, the Hervoken terrorised the New England town of Blackwood Falls in their attempt to leave the planet. (PROSE: Forever Autumn) On the same day, Torchwood Three had to deal with alien predators. (PROSE: Pack Animals)

On the planet of Verticulus, non-humans also celebrated Halloween. Jack-o'-lanterns with spider legs were placed outside as decorations and the inhabitants of Verticulus knocked on doors and asked, "Trick or treat?" but instead of candy, they expected baby brains. When Amy gave them Jammie Dodgers instead, they covered the TARDIS in toilet paper and eggs. (COMIC: Wholloween)

Other references
Professor Gilbert Horner described Beltane as the "greatest occult festival of the year, bar Halloween". (TV: The Dæmons)

Alistair Gordon Lethbridge-Stewart surmised the circumstances of his getting to the Dark Tower on Gallifrey as "like a cross between Guy Fawkes and Halloween." (TV: The Five Doctors)

Jilly Kitzinger excused her watch's flat battery by claiming that it was supposed to be changed once a year, say Halloween or Christmas, but then she was preoccupied with candy and presents respectively. (TV: Rendition)

In September 2018, Sheffield resident Dean mistook the Stenza warrior Tzim-Sha for someone wearing a Halloween costume early, thus nicknaming him "Halloween". Tzim-Sha responded by killing him. (TV: The Woman Who Fell to Earth)

Behind the scenes
Though there has only been one official "Halloween special" for Doctor Who or its televised spin-offs, there have been three episodes in total that premiered either on or around Halloween. The only Doctor Who episodes to officially air on Halloween were Planet of Giants (1), The Zygon Invasion, and, most recently, The Halloween Apocalypse. (Though the opening episode to Planet of Giants, in season 2, aired on 31 October 1964, this this was years before the predominantly American holiday began to be acknowledged in the United Kingdom.)

The penultimate series 8 episode, Dark Water, aired the day after Halloween on 1 November 2014. A review of the episode from cited it as being "everything you could ask for from a Doctor Who finale the day after Halloween", a reference to the episode's particularly morbid subject matter not being at all out of place around the holiday.

In spin-offs, the first episode of K9, Regeneration, was broadcast on Halloween in 2009.

Televised Doctor Who was set to enjoy its first set of Halloween specials via The Sarah Jane Adventures, which was planned to have four such specials appear some time after its truncated fifth series. As with the remainder of series 5, the specials were never produced due to the death of Elisabeth Sladen.

Doctor Who had its first Halloween special in The Halloween Apocalypse, the opening episode to series 13, which aired on 31 October 2021.