At the beginning of the episode, Janor the Grimlock mentions that Cole's costume is pretty great. This may be a nod to the fact that Michael Bailey Smith, the actor who played the Grimlock, also plays Cole's alter ego, Balthazor.
When the Grimlocks first attack the manor, at some point during the fight Piper freezes them with her toy wand. This is the only time throughout the entire series that Piper channels her freezing power through an object.
Piper is dressed up as Glenda the Good Witch from 'The Wizard of Oz'. Phoebe is dressed up as Elvira.
Title is based the term Halloween shortened from All Hallows' Even or Eve.
When the sisters were discussing their costumes and decorations, Phoebe picked up a witch riding a broom through an orange moon. When she first does this the witches head is to her right, audiences left, they then cut to Piper and then back to Phoebe and this is reversed. In the pilot Piper teases her about wearing a conical hat and flying around on a broomstick. In this story, she does all of that.