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)

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
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)

In September 2018, Sheffield resident Dean mistook the Stenza warrior T'zim-Sha for someone wearing a Halloween costume early, in September, thus naming him "Halloween". T'zim-Sha responded by killing him. (TV: The Woman Who Fell to Earth)