I'm not 100% sure I can tell what you're asking, since you're talking about multiple languages and are making what looks to be a very inaccurate assumption about Sundays, but I think the question is why "Sabado" (which is derived from "Sabbath") is the Spanish translation of "Saturday," despite a popular Christian tradition of observing Sunday ("Domingo" in Spanish) as the main weekly holy day.

There are a number of factors here, but the most important element to remember is that calling Sunday "the official day of rest and worship" without further qualification is hugely inaccurate. Different religions, and different groups within the same religions, observe different holy days. Sunday is merely one of the more widespread in western traditions (though Saturday is also a big one).

Now, as to "Sabado" in particular, it is derived from the Hebrew word "Shabbat" (Sabbath), which refers to Saturday. In many forms of the Jewish religion, Saturday is the holy day - not Sunday. If I remember correctly, it is specifically the time between the 6th and 7th sun-downs of the week. At any rate, the Hebrew word provided the base for the Spanish word.

Sunday ("Domingo") isn't left wanting for religious content either, if you hunt up its origins. "Domingo" is derived from Latin for "Lord's Day," so take that as you will.

On the 'Calendars and timekeeping' page of the FAQ, there's a section about 'Weeks, weekdays, and weekends', which includes some questions about Saturday and Sunday as the week-end.

Here's an answer I wrote to one of those questions, which explains that Saturday is the Jewish sabbath, which was continued by the early Christians. However, a few centuries after Jesus' death, the Christian Synod of Laodicea defined Christianity's holy day as Sunday, because it was the day of Christ's resurrection.

Therefore, while Saturday remained the sabbath day for Jews and Christians, the Christians moved their worship to Sunday.