Why is the Jewish Sabbath observed on Saturday?
Genesis, Chapter 1 provides the basis for the Jewish week and the understanding of its days – God creates on days 1 to 6 and rests on day 7; hence Shabbat is day 7.
The names of the days, familiar from Latin and Teutonic cultures, developed separately and were named for various gods – Sunday (Helios, the sun), Monday (Luna, the moon), Tuesday (Tua, a Teutonic goddess), Wednesday (Wotan/Odin, the chief Teutonic/Norse god), Thursday (Thor, Teutonic god of thunder), Friday (Freia, Teutonic goddess of youth), Saturday (Saturn, Roman name for Kronos, chief of the Titans).
Suffice it to say, the two systems for designating the days developed independently of each other (except, likely, for the designation of seven days as a week=one quarter of the lunar cycle).
To distinguish themselves from the Jews, Christians began to celebrate Sunday as the Lord's Day (the day Christ arose from the dead) rather than celebrating the Jewish Sabbath (although some Christian groups persisted in observing the Sabbath).