And it shall be a sign on your hand and a sign between your eyes, that through strength of hand the LORD brought us out of Egypt. Tradition teaches that this verse refers to tefillin. Tradition also teaches that one does not wear tefillin on Shabbat. Sfat Emet explains that during the week we need to be reminded of who we are and what we stand for. Therefore, at the beginning of the day, men literally tie themselves down. On Shabbat we enter a spiritual realm, remember who we truly are and need no such reminder.