Hello folks 
I'm currently in a conversation with a 7th Day Adventist who is telling me that it is basically a sin (violation of 4th commandment) that communal worship is held on Sunday instead of Saturday.
Now, I am trying to reach him as effectively as possible and I know that saying "Pope Gonzo XVII changed it at the Council of Youngstown, Ohio in 1245" will not hold much weight with him....nor with you....which is why I am coming here for help
Basically, the NT makes frequent references that the in the early Church, believers came together to worship on "The Lord's Day." But it does not specifically say "This was what you now call Sunday."
I think I can prove this from the Church Fathers, but again, he probably will not care.
So how is Sunday worship justified from a Sola Scriptura point of view?
Thanks in advance

I'm currently in a conversation with a 7th Day Adventist who is telling me that it is basically a sin (violation of 4th commandment) that communal worship is held on Sunday instead of Saturday.
Now, I am trying to reach him as effectively as possible and I know that saying "Pope Gonzo XVII changed it at the Council of Youngstown, Ohio in 1245" will not hold much weight with him....nor with you....which is why I am coming here for help
Basically, the NT makes frequent references that the in the early Church, believers came together to worship on "The Lord's Day." But it does not specifically say "This was what you now call Sunday."
So how is Sunday worship justified from a Sola Scriptura point of view?
Thanks in advance