There have been many kinds of days throughout time. Once Saturday stood for “the Sabbath,” a day of rest that was commanded by God. In Genesis 2:1-3, the Lord took a personal Sabbath, resting from the work of creating the world. When the Israelites left Egypt, God taught them to keep the Sabbath, and they did so until Christ took it away by His death on the cross (Exodus 20:8-11, 31:12-17; Colossians 2:13-17).
However, we are writing about the first day of the week, which is Sunday.
In the Bible, this day is called the “Lord’s Day.” It is the day when the saints come together to observe the communion and give into the treasury of the local church (Acts 20:7; 1 Corinthians 16:1-4). It is to be a day when men do what God expects, and not use the day exclusively for their own selfish pursuits.