"Catspaw", which originally aired on 27 October1967, was written with the intention of being a "Halloween treat" for the viewers.
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.