Answer: Samhainophobia

There’s disliking Halloween as a general holiday and then there’s samhainophobia when you’re downright—lock the doors, bar the windows, hide under the bed—terrified of Halloween.

The phobia’s name finds its roots in the ancient Celtic festival of Samhain (the precursor and celebratory grandfather of modern Halloween). The festival took place on October 31, was principally intended to mark the end of the harvest cycle, and featured many superstitions relating to the dead (and the symbolism of the earth “dying” before entering into the slumber of winter). Festival goers would frequently wear masks to ensure the spirits of the dead, which were believed to be active on the night of the celebration, would leave them alone.

While sometimes it’s hard to understand exactly how someone develops a given phobia (especially if it’s of something very innocuous like helium balloons or the like), it’s not so hard to understand how people would develop a phobia surrounding a night where the dead are purported to return to walk amongst the living and the living themselves make it a game to imitate the dead.