Shelves, ceiling fans, tops of picture frames, that kind of thing.
Everyone has family traditions, and one of mine is dusting before Advent :-).
ADVENT: The other Lent!!
Dust is a protective coating for furniture. Dust keeps the enemies away. It takes six months for a decent coat of dust to build up. So long as no one is foolish enough to stir up the air, dust is harmless.
At least, that's what my mom says. She must be right, too, 'cause no one's been near her side of the room in years and there's an impressive layer of dust on all of the horizontal surfaces.