Halloween is a tradition that is celebrated on October 31. People, usually children, dress up as monsters like vampires, werewolves and ghosts, as well as many other costumes. Some children also go trick-or-treating. (TXF: "Sanguinarium"; MM: "The Curse of Frank Black")

The event traces its origins to the Druids, who believed it was on Halloween that Saman summoned evil spirits. For the Celtic people of the British Isles, it was the last evening of the year, an especially propitious time for examining portents of the future, and a night when devils, witches and other evils roamed the Earth. (MM: "The Curse of Frank Black") The Druids referred to this day as Samhain or the Fourth Witch's Sabbath. (TXF: "Sanguinarium")

In 1946, young Frank Black celebrated Halloween by going trick-or-treating with three friends. Frank was dressed as a police officer, while his male companions were dressed as a cowboy, a hobo and a gangster. One of the houses they visited belonged to a Mr. Crocell, who told Frank about the origins of Halloween. Frank would later take his daughter, Jordan, trick-or-treating while she was dressed as Marge Simpson on Halloween of 1997. (MM: "The Curse of Frank Black")

In 1992, FBI Special Agent Fox Mulder commented that the unusual events taking place in Bellefleur, Oregon made it seem like Halloween took place everyday there. (TXF: "Pilot")

In October 1993, various FBI agents placed fake Halloween jack-o-lanterns around their offices in the J. Edgar Hoover FBI Building. (TXF: "Ghost in the Machine")

In 1996, Dana Scully pointed out that a patient had been born on Halloween. (TXF: "Sanguinarium")

In November 1996, Alex Krycek led Mulder and Scully to a courier carrying a diplomatic pouch that supposedly contained a bomb, but the agents discovered the pouch only contained a black rock. Before revealing the rock to Krycek, Mulder teasingly asked, "What did you get for Halloween, Charlie Brown?" (TXF: "Tunguska")

In December 1998, the ghost of Maurice argued with the ghost of Lyda that they should haunt people on Halloween, not Christmas Eve. Lyda pointed out that people are more hopeful around Halloween than around Christmas. (TXF: "How the Ghosts Stole Christmas")

In 2001, The The Lone Gunmen used some pentathol and a Halloween mask to gain information from Majestic 12 operative Morris Fletcher. (TLG: "All About Yves")

A home movie from the late 1960s shown in "Dreamland II" seems to suggest that Fox Mulder (dressed as Mr. Spock) and his sister Samantha (dressed as a fairy) are getting ready for Halloween. This is not directly stated though, and it could have been some other costume-related event.
Community content is available under CC-BY-SA unless otherwise noted.