According to Judeo-Christian tradition, Moses recieved the Commandments from God on Mt. Sinai, the 4th of which pretty much 'laid down the law' on what day of the week was to be observed as the sabbath.
"Remember the sabbath day, to keep it holy ... the seventh day is the sabbath of the Lord thy God..."

Early Christians, most of whom were Jewish, continued to keep the 7th day as sabbath, and it wasn't until the 4th century that Emperor Constantine, in a political act designed to appeal to pagans, unsupported by scripture, declared Sunday to be the official day of Christian worship.

I make this point not because it matters to me today, but because I am amused any time I hear a Christian claim that one who works or keeps their business open on what was originally pagan Rome's day of worship is violating the sabbath, when there's simply no biblical backing for this position. But then, that never stopped Christians from making all the other shit up either!

@p-nullifidian Actually, IF you can read Aramaic in which the ORIGINAL O.T. and Torah were written, it states that "on the tablets of stone, the finger of God inscribed ONLY the words Thou shalt keep the Sabbath Holy," ( quick translation from Aramaic to English btw).
Now I can't quite remember what the Hebrew/Jewish word for Sunday is I'm pretty darned sure it is NOT Sabbath.

@Triphid True, I cannot read a word of Aramaic, but I was taught that the Torah was written in ancient Hebrew which was later translated into Greek. In any case, if that's all the finger of God wrote, then either the tablets were too small, or Moses embellished!

@p-nullifidian Originally the more common language used around that region was Aramaic from which both Hebrew and Arabic evolved later.
The Diplomatic/Political language used at those times and before the Roman Empire came along Babylonia and documents were either written Cuneiform, Egyptian Hieroglyphs or a combination of both.
Historically, the 13 Tribes of Nomadic Hebrews, (plus there were also 12 Tribes of what was become Palestine ) had NO written script until approx. between 1,200 and 980 B.C.E. ( B.C. for those who still adhere to outdated Christian method of time dating.)

@Triphid You clearly are more knowledgable than I in this space, but it was my understanding that, prior to the rise of Rome, Greek language and culture dominated the region, and that the first translation of the Hebrew Bible was from ancient Hebrew into Greek, later referred to as the Septuagint.

@p-nullifidian I think you'll find that 'Greek Dominance' of the Mediterranean region only arose, somewhat, post the Philip- Alexander the Great era and the Ptolemaic Dynasties in Egypt in particular.
Ancient Roman Influences only really began to take effect around 450-400 B.C.E., and before that there were Phoenicians, the Carthaginians.
Egypt mainly concentrated on Egypt, the Sinai Peninsula and the lands from Egypt to the border at Kadesh, Greece did a few 'colonies' around the region pre the Conquests of Philip and Alexander, a few on the Italian lands, 2 on Sicily, 2 on Sardinia, Crete and a few other Mediterranean Islands plus of course the few on what was the coast of the Persian Empire at the time.
But, until then Aramaic WAS the recognized common language amongst the peoples inhabiting the lands of Palestine, Jordan, Syria, etc, and Babylonia was still the Diplomatic language used pre the Greek-Alexander Conquests of that region.