Halloween was an Earth holiday celebrated on the evening of October 31st. It was a holiday that celebrated witches, ghosts, mumbo jumbo, and other monsters that go bump in the night. It also allowed celebrants to dress up in costumes to trick or treat.
In 2267, Ornithoid explorers Korob and Sylvia, attempting to understand the galaxy, constructed a castle, garbed themselves as a wizard and sorceress, and used other imagery that suggested the holiday of Halloween to James T. Kirk and Leonard McCoy. Spock later theorized that the aliens attempted to telepathically scan the landing party, and "missed", somehow retrieving information from the subconscious fears of the explorers. (TOS: "Catspaw")
In 2364, when Jean-Luc Picard first entered his Dixon Hill holonovel, based on Earth in 1941, he was still wearing his Starfleet uniform. As a quick cover for his unusual attire, he explained to Jessica Bradley that he had "lost a bet." She replied, "Oh, well. At least you're ready for Halloween." However, Picard wasn't familiar with the term. (TNG: "The Big Goodbye")
Halloween was also referenced in the first draft script for ENT: "Fusion" (which had the working title "Equilibrium"). In that teleplay, Trip Tucker explained the traditions associated with Halloween to Fraht, recalling that it had always been his favorite Earth holiday while he'd been growing up. Tucker also remembered that, in his boyhood, he would usually dress up as either an astronaut or an alien at Halloween and that, one year, he and his brother had eaten so much candy that their mother had needed to take them to the hospital in order to get their stomachs pumped, a tale that intrigued Fraht. Though he wondered if getting stomachs pumped was traditionally part of the celebrations, Trip replied, "Not really…" and their conversation then went on to discuss Vulcan holidays. The crew of Enterprise was meanwhile planning to throw a Halloween party soon, so Tucker suggested to Fraht that, if his ship was still in the area when the party was held, he and some of his crew could attend, a plan Fraht approved of. When Trip pointed out Fraht would have to come to the party wearing a costume, they began considering what kind of costume he'd wear, Tucker recollecting his own childhood preferences for Halloween costumes. Since Fraht was actually an alien among Humans, Trip concluded by advising Fraht to simply come as he was, to which Fraht smiled.