IC 2118 or The Witch Head Nebula is my favorite astronomical Halloween object.
Image Data: Digitized Sky Survey; Color Composite: Noel Carboni
This lunar eclipse looks very much like Pumpkin Moon. Carved pumpkins is one of the characteristics of our Halloween celebrations.
Credit & Copyright: Sebastien Gauthier
The Perseus Cluster's X-Ray Skull is a heavenly example of another symbol of Halloween namely skulls.
Credit: A. Fabian (IoA Cambridge) et al., NASA