ASBI Pocket Beit Midrash: Aruch HaShulchan Orach Chaim 242:2

The holiness of Shabbat is higher than all other holiness, and its blessings are above all other blessings. Therefore, it was sanctified and blessed from the beginning of creation, as it says, "And God blessed the seventh day and sanctified it." And this is the source of blessing to all the other days of the week. Therefore, Israel was commanded regarding Shabbat in seven different parshiot in the Torah, in Beshalach, Yitro, Mishpatim, Ki Tissa, Vayakhel, Emor, Va'Ethanan, to show that all seven days of the week are dependent on Shabbat. Therefore, we mention Shabbat every day when we say in the Song of the Day "First Day towards Shabbat," "Second Day towards Shabbat" and so with them all.