To dream of Halloween, signifies death and the underworld. Halloween also represents the temporary adoption of a new persona where you feel less inhibited and more comfortable to freely express yourself.
The idea of asking and receiving, as in trick-or-treating
The idea of costumes or hiding your identity
The idea of spookiness, haunting, or things on "the other side"
Something you associate with the Halloween time of year, such as candy or Autumn leaves
An event around Halloween time, either one from the past or one that you expect, desire, or fear