That is one of the topics that is debated among Christians who keep Saturday, the seventh day of the week, as the Sabbath, and those who worship on Sunday, the 1st day of the week, often referred to as the Lord's day. Any of you who dig deeply will understand why the Seventh Adventist scores big on the correct Sabbath Day. It's literally all over the Bible.
We don't need to debate what day is the Sabbath. The Jews are the best at keeping the correct day, from the very beginning when God told them to, so we know what day they keep their Shabbat. What has been interesting to me, over years of study, is really the fact that even from the very beginning, God instituted the seventh day rest. Take a read, Genesis 2:2-3. We know God is Spirit, and He doesn't need rest, but He did so as an example for mankind to follow, just as Jesus didn't need to be baptized, whether one argues its symbolic, or not, in washing away of one's sins.
Another good example that comes to mind is when God told the Israelites to observe the Sabbath rest, out in the wilderness, He clearly told them not to gather manna on the Sabbath Day of rest. And those who did, were not only disappointed there were none to gather, but the Lord was very displeased. Please read Exodus 16:14-29. Also, on the Sabbath, the manna did not spoil.
Of course, today, in the New Testament era, we take examples of how Jesus kept the Sabbath, as there's no more burdensome rule of Mosaic laws, and there's to much mention, so you want to read on how Jesus and His disciples kept the Sabbath, and pray for God to open your heart, and mind to that understanding.
The key is that the Sabbath was never removed as part of the 10 commandments. There's a simple reason why. God testing our faithfulness. As He instructed His people in the Manna example, as well as when He told Moses to talk to the rock, but Moses struck the rock instead. We humans may think, what's the big deal, Sunday is the day mainstream believers all worship. Do we really want to follow the majority of the Christian world who followed their denominations and leaders, just because the early church leaders did so to change Saturday to Sunday under Emperor Constantine during Roman time?
Yes, by now you can tell that I believe the Sabbath :), since it was meant to be a day of rest since the creation of the world, even before it was the 4th commandment, written by God Himself, because He promised us its a day He will bless and hallow it. (Exodus 20:8-11)
However, I must confess, I don't always keep the day as a rest to the Lord. I don't always tend to the things He would like me to do, or take more time to just talk to Him. I've been busted by my wife a few times watching college football, or YouTube special interest videos. Yea, not too good a head of the household model.
My wife prefers worship on Sundays because she likes praise worship at the larger churches. LOL. So yes, I have 2 worship days. So I need to ask the Lord to teach me how to be better Sabbath keeper, as I don't have a supporting church, or a group of believers that I can worship with regularly. Just my supportive wife, kids, and a small rag number of believers, and folks we help out in life, who join us for an hour of praise worship, scriptural study, and prayer on the Sabbath. Hope my sharing helps. Sorry so wordy. God bless you!!