Before I answer, let me pose YOU a question: Do you really think that wether you worship on Sunday or Saturday is a serious concern? I think that having a "day of rest" dedicated to the Lord is more important rather than the day. The tradition you trying to use as an example is a poor one to argue.
Yep! Genesis 2:3, [And God blessed the seventh day and made it holy]..... He never changed his mind and said, "Oh, lets do Sunday now."
Exodus 31:12, [Then the Lord said to Moses, "Say to the Israelites, 'You must observe my Sabbaths'. This will be a sign between me and you for the generations to come so that you will know that I am the Lord, who makes you holy".]
And in verse 16-17, [The Israelites are to observe the Sabbath, celebrating it for the generations to come as a lasting covenant. It will be a sign between me and the Israelites forever.]