Nanny'd told Tiffany that witches stayed in bed on Soul Cake Tuesday, so the people could pretend witches didn't know about the dressing up in pointy hats and removable warts that they got up to. In Tiffany's case, it seemed a weak excuse, since she'd tricked and treated on these hills since she was two and everyone knew about it.
Tiffany didn't care to stay in, so she sewed herself a ghost costume in good time out of some old sheets. She'd seen one in Nanny's closet, too. It simply wasn't in a witch, to do what she was told.