Halloween is the night when the world of the dead is closest to the Mortal world.
Halloween Night is when the borders between the spirit world—the Nevernever—and mortal world are at its weakest. Halloween is also the night that the Immortals feed refuel, run free. It is when "the locked stasis of immortality becomes malleable"—they can take in energy, add new power to their mantle.
Halloween is a conjunction, a place that is in time, rather than space. It ends with the first natural morning birdsong, or rooster. Sundown is when the immortals come out to play. Trick-or-Treating comes from immortals tricking each other out of power each year on Halloween. Everything in this world is mortal on Halloween, the world of the living and the one of the dead are closest. It is the night in which Immortals can be killed or created by the Darkhallow. Bob's knowledge of this makes him a threat to immortals—and Mab knows it, there are very few people knowledgeable about the topic.
Backstory of HalloweenEdit
The second or third Merlin of the White Council started the Halloween costume. The reason that people started wearing masks is that any hungry immortals might think twice. They could never be sure that their prey was not another immortal in disguise.
Halloween in the seriesEdit
In Grave Peril, when Bianca St. Claire and Leonid Kravos—the Nightmare—tortured many ghosts, it was for the purpose of creating turbulence to further breaking down the barrier already weakened by Halloween. The fact that it was Halloween night is also why the Nightmare was able to steal some of Harry Dresden's power, and why Dresden was able to steal it back later.
In Dead Beat, because Halloween was so close, Harry Dresden could sense the restlessness of the ghosts in Graceland Cemetery where he was meeting Mavra. Dresden said that all of his jobs start looking the same around Halloween.
Bob and Dresden discuss the Word of Kemmler and what would happen if the the Heirs of Kemmler got a hold of the book. Dresden realizes that it's no coincidence that it's nearly Halloween. Like when the Nightmare was hunting down Dresden's friends—he and Bianca eroded the Halloween-weakened barrier by torturing ghosts. Bob said it's not necessary to torture ghosts to weaken the barrier; rituals and sacrifices will do.
In Cold Days, Bob told Harry Dresden the relevance of Halloween. That it is a conjunction, one that is the only time an immortal can be killed, and the only time the Darkhallow can be performed to make a mortal immortal. Maeve and Lily were able to be killed because it was Halloween.