Fallingblood, with all due respect, I think an argument can be made that the Bible does say when the sabbath is. The argument is passive and implicit rather than active and explicit, that is certain. But the days of the week seem to have always been named the same in Hebrew (as opposed, say, to the months, which have changed names at least once or twice). The days of the week have always been numbered and not named: so whether in Biblical Hebrew of the earliest strata, or later Tanakh texts (to say nothing of Rabbinic Hebrew through to modern Hebrew), Sunday is always yom rishon ("First Day"), Monday always yom sheni ("Second Day") and so on through to Friday, which is yom shishi ("Sixth Day"); but Saturday is consistently Shabbat (the sabbath day). This is absolutely consistent, without variation, regardless of source authorship or presumed dates of origin.
Which also makes sense, if you think about it, in terms of narrative instructions, also. The Fourth Commandment, for example, as it is laid out in Exodus, begins zachor et yom hashabbat lekodsho; sheshet yamim ta'avod v'asita kol melachtechah, v'yom hash'vi'i shabbat l'YHVH elohechah.... "Remember the day of Shabbat, so that you may sanctify it: you shall work for the six days [of the week], doing all your labors, but the seventh day is Shabbat, for YHVH your God...." The phrasing makes it clear that the days numbered one through six are the work week, and the seventh day is Shabbat. The version of the commandment in Deuteronomy uses essentially the same construction, with slightly different phrasing, but the same presumption seems clear. If it were not so, why would there never be a commandment or other laws given specifying which of the seven days is Shabbat? After all, observance of Shabbat was a matter involving considerable obligations, the abrogation of which could (in theory) result in the death penalty. Why would they be vague about this unless it was absolutely manifest and irrefutable-- as in, for example, the days of the week all being sequentially numbered and not named, save only for the day called Shabbat-- which day they considered the seventh day, the day of Shabbat?
I'm not trying to be insulting or quarrelsome-- you know that. But I also do think it's fairly difficult to support the argument that the day of Shabbat meant anything but the day called Shabbat-- the seventh day, which is Saturday on the Roman calendar (there is absolute and unquestioned clarity on this equivalence in the Talmud, also). I'm certainly not saying that Christians or anyone else shouldn't celebrate their sabbaths whenever they like, on whatever day they please. After all, they are not commanded: Torah was never meant for non-Jews. So it matters not a whit what they do. But it seems awfully clear to me that the original sabbath as it was meant and intended in the Tanakh was on the day we call Saturday.
