the early Christians would get together on every sunday evening and eat a feast together. they called this day the Lord's day to commemorate Christ's resurrection on sunday morning. at these meals they would break bread and pass wine to remember the atonement on the cross and the elders among them would teach or answer questions. there would be prayer and usually an offering of some kind for other members of the body. as time went on and Christianity became the religion of the empire buildings were erected to worship in and priests and bishops became officers that preached in the morning on those days. people frequented the church buildings so that they often had people worshipping every day of the week, but they collectively would gather sunday mornings to give service. church as we know it today has existed for less than 1000 years, and most church practices even less than 100. but to say "sunday worship is paganism" is a misuse and abuse of history. yes paganism generally worships the sun, but the first day of the week has not always been called that. the Jews and Christians of Christ's day did not call it sunday, they did not associate it with the Sun at all. it was simply the first day of the week.
many teachers and cult leaders have claimed that sunday worship is pagan to divide the Church. none have been able to show its development historically, because it didn't happen that way. Paul tells us that days do not matter. festivals and worship days mean nothing to God anymore. Christ and His sacrifice are all that matter now. Let us rejoice in that together.
May God be the Judge