The early church met on Sunday, so most Christians follow them by example. Some, such as the Seventh Day Baptists may meet on Saturday and that is their right if they wish to do so, as long as they do not try to turn that into a command for other believers under the New Covenant. That would be legalism which usually would turn into a works salvation. The Seventh Day Baptist web page reads: "We lovingly observe the 7th Day Sabbath. It is a blessing, not an obligation."
"AND NOW ABOUT the collection in aid of God's people: you should
follow my directions to our congregations in Galatia. Every Sunday each of you is to put aside and keep by him a sum in proportion to his gains, so that there may be no collecting when I come." (1 Cor. 16:1-2 New English Bible)
https://www.katapi.org.uk/NEB/NEB.html#NT
But on
the first day of the week, at early dawn, they came to the tomb, taking the spices that they had prepared. (Luke 24:1, NRSV)
"When it was evening on that day,
the first day of the week, and the doors of the house where the disciples had met were locked for fear of the Jews, Jesus came and stood among them and said, 'Peace be with you.'” (John 20:19, NRSV)
"
A week later his disciples were again in the house, and Thomas was with them. Although the doors were shut, Jesus came and stood among them and said, 'Peace be with you.'"
(John 20:26, NRSV)
"On
the first day of the week, when we met to break bread, Paul was holding a discussion with them; since he intended to leave the next day, he continued speaking until midnight." (Acts 20:7, NRSV)
"I was in the spirit on
the Lord’s day, and I heard behind me a loud voice like a trumpet" (Rev 1:10, NRSV)
"When you come together, it is not really to eat
the Lord’s supper." (1Cor 11:20, NRSV)
There is no command to meet on Sunday, or the first day of the week, but that is when the early church met. If "the Lord's supper" refers to Jesus Christ, it is natural that "the Lord's day" would refer to Jesus Christ, not to Moses and the Old Covenant.
Early disciples were Jews and to reach Jews, of course they'd go to the Jews meeting on their Sabbath. The Sabbath was for Jews, the Old Covenant. So, following the example of the early believers in their meetings, it was on Sunday, the first day of the week and most Christians do likewise.