Sunday Sabbath is not in the Bible
This is true. We celebrate the Lord’s Day because that is the Day He chose.
Jesus IS our sabbath rest. We find our rest in Him.
9 So then, there remains a sabbath rest for the people of God; 10 for whoever enters God’s rest also ceases from his labors as God did from his. Let us therefore strive to enter that rest, that no one fall by the same sort of disobedience. Heb.4
Mark 16:9 " Now when he rose early on the first day of the week, he appeared first to Mary Mag′dalene"
John 20:19 [ Jesus Appears to the Disciples ] On the evening of that day, the first day of the week, the doors being shut where the disciples were, for fear of the Jews, Jesus came and stood among them and said to them, “Peace be with you.”
Acts 20:7 “On the first day of the week, when we were gathered together to break bread, Paul talked with them, intending to depart on the morrow…”
1 Corinthians 16:2 On the first day of every week, each of you is to put something aside and store it up, as he may prosper, so that contributions need not be made when I come.
The early Church gathered on the Lord’s day to celebrate Jesus’ resurrection, and His appearance to the disciples. Since all the first Christians were also Jews, they celebrated the Sabbath as well as the Lord’s Day.