It’s the 31 October during Next Book, how is Harry celebrating? Trick or Treating is probably somewhat limited, costumes are more likely on the obvious cute rather than gothic as it would trigger far too much PTSD.
Harry has the Castle, but Murphy, Thomas and Justine are not there to help him celebrate. Eb is likely still hospitalised and all his Warden friends are dead, stuck in an alternate universe, or being lickspittles for the Senior Council.
The Alphas are available and likely Butters has healed.
In the circumstances I think Harry hosts a Halloween Party for his neighbourhood, in his Great Hall as a safe place with plenty of food brought in from the Never, Never, and food such as candy apples, pumpkin pies, chocolate etc that can be taken away and eaten over the next few days, by hungry families.
Harry wouldn’t really want to do this, but would do it for the sake of Maggie and that she needs to get used to her new neighbours, and they to them. Remember his neighbours knew Mouse, but not him and Harry needs to rectify this.