We find in the Torah three mitzvot that are referred to as a "sign" or "covenant" between God and the Jewish people. One is Shabbat, another is Brit, and the third is tefillin (phylacteries).
The presence of two of these "signs" is constant: On weekdays the two are the Brit and tefillin; On Shabbat the two signs are the Brit and Shabbat. (This is one reason why we do not wear tefillin on Shabbat).
An allegorical hint to this idea can be found in the Hebrew word Shabbat. Shabbat is spelled Shin, Beit, Tof. The letters form an acronym for "Shabbat Bimkom tefillin" — Shabbat in place of tefillin." In reverse it reads: "Tefillin Bimkom Shabbat." - tefillin in place of Shabbat."