According to the Hebrew Bible (Torah) Sábado (Shabbat) is the Jewish rest day (equivalent to Sunday for Christians and Friday for Muslims). This was the seventh day of creation, following the days that were simply enumerated in the Bible as the 1st, 2nd, 3rd, 4th, 5th, 6th. This serves as the basis for the Hebrew names for the days of the week (still in use in modern Israel), which are literally translated as "the first day" (Monday), "the second day" (Tuesday), etc.:
||sixth day (day of prayer)
The Hebrew convention was adopted by the Arabs (with the exception of the name for Friday, which is the day of prayer for Muslims), and inherited from them by the Christians in the Iberian peninsula.
The alternative names for the days of the week, those used, e.g., in Spanish, French, or English have their origin in the Roman tradition, Solis dies, Lunae dies, Martis dies, Mercurii dies, Jovis dies, Veneris dies, Saturni dies, referring to the Sun, Moon and the pre-Christian Roman gods. The exception here are the names for Saturday and Sunday: the former adapted from its Biblical/Hebrew form, the latter meaning Domini Dies ("the day of the Lord") - this is also true for the names of these days in Portuguese.
See, e.g., Religion and the seven days week for more details.