Halloween is a holiday celebrated on the evening of October 31st, emphasizing on witches, [[ghost]s], and other monsters that go bump in the night.

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.
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