Cite?
If worshiping on Saturday versus Sunday was so critical why did the earliest Christian churches do this?
Please show me where they worshiped on Sunday. You obviously can't from scripture because every passage that the translators have translated as "First Day of the Week" actually says in the Greek....."On one of the Sabbaths" or is related to the Sabbath. That's the whole point! [Matthew 28:1][Mark 16:2-9][Luke 24:1][John 20:1][Acts 20:6-7][I Corinthians 16:2]. Show me in the Greek where any one of these scriptures is properly translated "First day of the week'!
Besides the secular sources already given, read:
ICor. 11:21 for as you eat, each of you goes ahead without waiting for anybody else. One remains hungry, another gets drunk.
What was happening was on Sunday evening when they regathered the more affluent Christians were eating and drinking prior to the poorer Christians who could not come until later. The poor came and were only left with a symbolic (small) portion.
22. Don't you have homes to eat and drink in? Or do you despise the church of God and humiliate those who have nothing?...