Hallows:
1. To make or set apart as holy.
2. To respect or honor greatly; revere.
3 to consecrate anew, as after a desecration.
Deathly:
1. having the physical appearance of death
2. causing or capable of causing death; "a fatal accident"; "a deadly enemy"; "mortal combat"; "a mortal illness"
So to me, the first question I would ask is what in the HP series has been desecrated. The first thing that comes to mind is the soul of Voldemorte. book 6 made a huge deal out of just how much damage Voldy has done to his soul by creating 7 horcruxes. The other point is that the word is deathly, and not deadly. To me, I think there is a difference. The meaning of deathly is having the appearance of death . To me, that means, like death, but not actual death. There was a huge point about faking death in book 6 as well. I'm not sure, but could this have to do with the purification of Voldy's soul? I was thinking on a superficial level, it could mean to make VOldy's soul whole again, but releasing all 7 parts from this existence into the afterlife.
Though of course there's also the connection to the word Holloween, and relating to the world Hollow. as in Godric's Hollow. Of course all things relate back to Gordric's hollow. and it would also makes sense the word deathly would be in there, since Harry's parents died there.
Then there's the idea of Halloween. This thing started on halloween, and it would make sense that it ends there. And in many cultures, Halloween is considered sacred, because it's the one time where magic is most potent, and the spiritual world can make contact with the physical. Just a few thoughts.