Oh, there were no days off, any more than there are days off for the robots in "Westworld". Wouldn't even make sense as a concept in that context. Not just a useful comparison, either, since (long story) They were using that show to allegorically tell us how it used to be.

There is no clear and universal consensus on when the idea of a "sabbath" day came into human culture, but I found this passage quite intriguing:

The earliest extrabiblical attestation of Sabbath might occur in a 7th-century BCE ostracon discovered at the ancient fortress of Mesad Hashavyahu, which could refer to a servant doing certain kinds of work "before Sabbath" (lpny šbt).

In another incredibly long story, it turns out the whole world changed drastically around 600 BC, the same dating as this origin of the sabbath. It goes back once again to the Anunnaki, and the settlement of what is loosely known and widely misunderstood as the "war in heaven". The "peace treaty" went into effect at that time.

So in that scenario, the sabbath just came into being along with a couple of hundred other things that humans started thinking up, such as the very idea of writing down history. Big changes all around.