The outfit, you need to buy the halloween themed furniture and set it up in your room.
Then for Gwen's halloween event you need to go to the dungeon at night (fix it if you haven't)
As for the decoration, it's hidden in Luna's demesne's tunnels.
Shame I can't hide text in here, so if you want to find it on your own you can stop reading.
Enter Tunnel > Go West > Go all the way South > Go all the way West