In faery tales, it is the place where unsuspecting travelers cross over from reality into Fey. It is the space in between normal and surreal. Dusk, twilight is a time when 'Tween is really apparent even to people who are not paying attention. Midnight, crossing over water, Samhain when the shroud or veil is lifted and everything seems to be going sideways. Tripping seems to almost induce this feeling. It's a time or place that brings the hair up on the back of your neck, and there's probably a good reason for this, so just shrugging it off won't do you any good. Keep your eyes and ears open.