"va-y'kadaysh ohsoh" - (and He made it holy) [B'rayshis2:3]
Its holiness is that it is a day of rest. If you do not believein the Torah, Shabbos does not make sense. Why should you stopworking on a busy day? However, for a person who believes in theTorah, Shabbos is a source of blessing, and it is the basis ofthe world.(Ramban)