A day that God claimed as His own
Nine of the ten commandments govern what a man does. One governs when he stops. It names a particular day rather than a convenient one, and it gives the reason plainly: six days of making, then rest.
“Remember the sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: But the seventh day is the sabbath of the LORD thy God … For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.”
Exodus 20:8-11, KJV