Halloween is a holiday celebrated on the evening of October 31st, emphasizing on witches, ghosts, and other monsters that go bump in the night. The celebration of this holiday seems 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 like wizard and sorceress, and used other imagery that suggested the holiday of Halloween to the 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")
- The aptly themed episode "Catspaw", which originally aired on October 27, was written with the intention of being a 'Halloween treat' for the viewers during its original airing in 1967.
In 2364, when Jean-Luc Picard first entered his Dixon Hill holonovel, which was based on Earth in 1941, still wearing his Starfleet uniform. As a quick cover at the sight of Jessica Bradley, as he entered his private office, he noted that he was wearing such an unusual uniform because he had "lost a bet". She replied, "Oh well. At least you're ready for Halloween" -- a term Picard was not familar with. (TNG: "The Big Goodbye")