Halloween

Halloween Night is when the world of the dead is closest to the Mortal world.

Description
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 immotality becomes maleable"—they can take in energy, add new power to their mantle.

Everyone, everything standing in this world is mortal on Halloween. It is the night in which Immortals can be killed.

About / Details

 * Halloween is Harry Dresden's birthday.  (previous refs? before Dead Beat?)
 * Halloween is a Conjunction: a places that is in time, rather than space.
 * Black magic on Halloween meant that Necromancy was likely going to be used.
 * Cowl was probably the only Heir who knew that the Darkhallow had to be on Halloween Night.
 * Bob knows that Immortals can be killed on Halloween Night making him a threat to immortals—Mab knows he knows.
 * There are only a couple of people alive who know that Immortals are vulnerable on Halloween and the Immortals will keep it that way.
 * Halloween ends with the first natural morning birdsong, or rooster.

Halloweens in the Series

 * The events of Fool Moon and Grave Peril occur around or on Halloween.
 * It was Halloween night that the Heirs of Kemmler, specifically Cowl, attempted a Darkhallow spell to devour ancient spirits in order to gain godlike powers.

Backstory of Halloween
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.

In the Series Notes
In Grave Peril, Bob didn't think that the crazy ghosts freaking out all at once around the city had anything got do with Halloween.

In Dead Beat, because Halloween was so close, Harry could sense the restlessness of the Ghosts in Graceland Cemetery where he was meeting Mavra. Harry said that all of his jobs start looking the same around Halloween.

In Cold Days, Bob told Harry the relevance of Halloween. That it is a Conjunction, one that is the only time an immortal can be killed. And, it is the only time the Darkhallow can be performed to make a mortal immortal.