שמות כ, ח ז כוֹר אֶת-יוֹם הַּשַּב ת לְקַּדְ שוֹ
Contrary to the way of the world, we do not give names to each day of the week, but we count from Shabbos, i.e. day one to Shabbos, היום יום ראשון בשבת. Thus, the Medrosh explains that one does not put on Tefillin except שבתב, which means that we only put on Tefillin in the days that we count to the day of Shabbos, but not the day of Shabbos itself היום יום שבת קודש.
In the Torah we do not find a reference to the day in the week, only to the day within the month. Perhaps, since the days of the month are set by Chaza”l (by declaration of the new month), whereas the days of the week are set by HaShem. Thus, in order reinforce the concept of the Jews’ establishing the count of the days of the month, the Torah refrains from counting the days within the week.
