Then why didn't the early Christians celebrate their Lord's Day on Saturday?
They did. They also continued to celebrate the Feast Days of the Lord outlined in Leviticus 23.
Polycarp, Bishop of Smyrna and disciple of the last living Apostle, John, was still celebrating Passover well into the 2nd century. He was giving the Roman Church fits because of this....but don't you think Polycarp would have got the word about "Easter" from John the Apostle?
You can verify everything I have just said. Again I'll ask just so it sinks in. Why do you suppose Polycarp was not celebrating Easter and in fact was still celebrating Passover well into the 2nd century? He told Anicetus, the Bishop of Rome he had always observed Passover with John, the Lord's Apostle.....and the other Apostles.
As you know, Polycarp was martyred, killed by Roman soldiers for his beliefs....155 A.D.