Calvin's Commentaries, Vol. 4: Harmony of the Law, Part II, tr. by John King, [1847-50], at sacred-texts.com
12. Six days thou shalt do thy work, and on the seventh day thou shalt rest: that thine ox and thine ass may rest, and the son of thy handmaid, and the stranger, may be refreshed.
12. Sex diebus facies opera tua: die autem septimo quiesces, ut quiescat bos tuus, et asinus tuus, et respiret filius ancillae tuae, et peregrinus.
12. Six days thou shalt do thy work. In this passage the incidental use of the Sabbath is again referred to, although it is no inherent part of its original institution, viz., that by its means the family also and the cattle shall be benefited. There is no impropriety in reckoning this amongst the other blessings which enhance the value of the Sabbath, although it is a portion of the Second Table. And we know that this rude people required to be attracted by every possible means to present cheerfully to God the worship due to Him. The sum therefore is, that they were thus to testify not only their piety towards God, but also their kindness towards their servants. I have already shewn that their authority as masters was to be exercised in moderation by them, if they were mindful of their former condition:, since they also had been servants in Egypt. If any one should suppose that the argument does not hold good, because; they were oppressed by cruel and dreadful tyranny, the reply is easy, that so much the better could they determine from their own feelings how detestable and intolerable a thing cruelty is.