Each of the days of the week is considered to be the partner of another day of the week, apart from Shabbat, whose partner is Israel.

Rabbi Shimon bar Yohai taught: Shabbat said to the Holy One, blessed be He: Master of the Universe, each day has a partner, but I do not have a partner. How can that be? The Holy One, Blessed be He, said to it: The congregation of Israel is your partner. When Israel stood before Mount Sinai, the Holy One, blessed be He, said to them: Remember what I said to Shabbat, the congregation of Israel is your partner: “Remember the Sabbath day, to keep it holy” (Exodus 20:8). (Bereshit Rabba [Theodor-Albeck] 11)