Choosing any day that we wish to worship on is similar to Cain’s sacrifice of choosing any sacrifice that he wished instead of obeying what God said to do.
The Ten Commandments were given at Mount Sinai in Exodus 20. Yet before this, God instructed Moses and the Israelites to keep the Sabbath. They knew to keep it Holy before the Ten Commandments had yet been given. One example is below in Exodus 16 where they were to gather twice as much food on Friday so that they could rest on Saturday:
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.
Exodus 16:29,30
So the people rested on the seventh day.
The Sabbath is a sign between God and us that He is the one who sanctifies us (makes us holy, frees us from sin):
Moreover also I gave them my sabbaths, to be a sign between me and them, that they might know that I am the LORD that sanctify them.
Ezekiel 20:12