How long have the days of the week been in continuous use? For example, 2.8 million days ago (400,000 weeks ago, circa 5650 BC), were there people who called it Thursday (or their tongue equivalent) and there has been a Thursday every seven days since.
Was the bible ("… so on the seventh day, he rested …") written after the number of days in a week had been established? Or did the bible set the number of days in a week at seven?
Wikipedia provides a lot of information. It seems interesting that the word for ‘seven’ in several languages, including Proto-Indo European, is borrowed from Semitic.
Are you asking whether the number of days elapsed between some Jewish Thursday 2500 years ago and today (Thursday) is a multiple of 7? I don’t know. I did find it interesting that on completion of Magellan’s round-the-world trip lasting three years, the ship’s supernumeray scholar, Antonio Pigafetta, was confident enough in his record-keeping to dispatch an emissary to the Pope explaining the “Date Line.” (A lesser man might have assumed he lost a day in the arms of a Filipino nymphet!)
Should be noted that, when the Catholic Church reformed the calendar under Pope Gregory XIII in 1582, the days were not affected. Thursday, October 4 was followed by Friday, October 15.
In general, the explanations in that particular section of the bible are post-hoc justifications for things that were already known and taken for granted at the time it was written. (Or, worded less charitably, “just-so stories”.)
Septimus’ link (if correct) provided the answer I was looking for, to wit:
A continuous seven-day cycle that runs throughout history, paying no attention whatsoever to the phases of the moon, was probably first practiced in Judaism, dated to the 6th century BC at the latest.
The days of the week in Hebrew are named with the numbers from 2 to 7; the exception is “Yom Rishon”, the first day (not day one) but the rest are “Yom Sheni”, day 2 (from shenaim, 2), “Yom Shelishi” (from shelosh, 3) all the way to “Yom Shabat”, the seventh, Sabbath day, when God rested.
Supposedly the Hebrew names reflect the original names God gave to each of the seven days of Creation, as recorded in the Book of Genesis.
I only have Wikipedia to go on, not being an expert in this - but it appears from what I can gather there that the description of the origin of the days of the week in Genesis might be about a century newer than that.
Fitting that the first reply to the OP comes from Septimus.
It looks as if it was the same Babylonians that gave us 360 degrees in a circle:
Seven days is relevant for a much more direct reason than that: It’s (approximately) the time between the main phases of the Moon. If the Moon is full tonight, then it’ll be half a week from now, and new a week after that, and so on.
Approximately, at least. If that’s your standard, then weeks will usually be seven days but sometimes eight. Apparently the Jews were the first to standardize on weeks always being seven days, even though that doesn’t keep perfect time with the Moon.
Seven day weeks aren’t universal. The Romans, for example, used an eight day week.
The OP should correct me if I’m wrong, but I took the question to mean something like, St. Patrick’s Day was on a Saturday this year (2018). September 11 2001 was a Tuesday. Pearl Harbor was bombed on a Sunday. The Declaration of Independence was adopted on a Thursday. So, how far back can we go like this and be reasonably certain that the people on that day were using the equivalent name for what we’d call that day? Would Columbus have called the day he stepped onto some Bahaman shore a Wednesday? Would Charlemagne have recognized he was crowned Holy Roman Emperor on a Friday?
Myself, I’ve wondered which day of the week the K-Pg impactor struck the Yucatan, or which day of the week the Big Bang occurred.
If you want to read a great little book that covers this and many other topics on the history of time keeping try Isaac Asimov’s The Clock We Live On.
Hmm. “ראשון” (from ראש = head, beginning) is a standard ordinal expression in Hebrew. So you mean 1 through 6 or 2-6. Is the myth that Proto-Semitic numerals literally properly named days of creation? Also, Sabbath by no means means “seventh”.
Is it attested that God spoke Hebrew and not Greek or Akkadian? 
Well, of course, the text we now have was compiled around then, but it’s a compilation of earlier texts, which in turn record oral traditions. So the creation account which explains the seven-day cycle could be much older than the text of Genesis known to us.
Does it seem clear, though, that the rigid seven-day pattern forming a week had already been in existence for some time before the first telling of the first version of the story that later became Genesis 1?
As did the Beatles!