in giving the sabbath command God said:
Exodus 20:8(KJV)
Remember the sabbath day, to keep it holy.
the verse implies that the COI was already keeping God's sabbath.
This is the first time the Sabbath was taught to Israel:
Exo 16:23 And he said unto them, This is that which the LORD hath said, To morrow is the rest of the holy sabbath unto the LORD: bake that which ye will bake to day, and seethe that ye will seethe; and that which remaineth over lay up for you to be kept until the morning.
Exo 16:24 And they laid it up till the morning, as Moses bade: and it did not stink, neither was there any worm therein.
Exo 16:25 And Moses said, Eat that to day; for to day is a sabbath unto the LORD: to day ye shall not find it in the field.
Exo 16:26 Six days ye shall gather it; but on the seventh day, which is the sabbath, in it there shall be none.
Exo 16:27 And it came to pass, that there went out some of the people on the seventh day for to gather, and they found none.
Exo 16:28 And the LORD said unto Moses, How long refuse ye to keep my commandments and my laws?
Exo 16:29 See, for that the LORD hath given you the sabbath, therefore he giveth you on the sixth day the bread of two days; abide ye every man in his place, let no man go out of his place on the seventh day.
Exo 16:30 So the people rested on the seventh day.
If they kept the Sabbath before this then Moses would not have to tell them what to do and not do on the Sabbath. This was clearly a new thing to Israel.