Halloween was a Earth holiday celebrated on the evening of October 31. The term Halloween, and its older spelling Hallowe'en, was shortened from All-hallow-even, as it was the evening before "All Hallows' Day." The holiday originated as a day of religious festivities in various northern European Pagan traditions, though it evolved into a holiday emphasizing witches, ghosts, and other monsters that go bump in the night. The celebration of this holiday seemed to have fallen out of popularity by the 24th century.
In 2267, alien explorers Korob and Sylvia, attempting to understand our galaxy, constructed a castle, garbed themselves as wizard and sorceress, and used other imagery that suggested the holiday of Halloween to Kirk and 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, which was 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" -- a term Picard was not familiar with. (TNG: "The Big Goodbye")