XS> Please provide the scriptural citation for that assertion being changed to the Pagan day of Sunday.
"And on the first day of the week, when we were assembled to break bread, Paul discoursed with them, being to depart on the morrow: and he continued his speech until midnight." (Acts 20.7)
"On the first day of the week let every one of you put apart with himself, laying up what it shall well please him; that when I come, the collections be not then to be made." (1 Corinthians 16.2)
The L-rd G-d in Genesis chapter one commanded a day to begin at evening and continue until the following evening. The first day of the week begins at nightfall after the L-rd's Day, Shabbat, is complete. Traditionally there is a gathering for an evening meal called an Oneg. Collecting money would not be a proper thing to do on the L-rd's Day, the Shabbat. I hope this clarifies all misunderstandings.
Nowhere in the cited verses do we find a command from G-d
b'shem Y'shua
to change the L-rd's Day from the commanded Shabbat to the first day "Sunday"
So it would be left to the following day, after dark and thus on the first day.
I don't think there is any misunderstanding at all about your Pagan Pontiff Constantine changing the Church to worship on Sunday theory. Its complete and 100% pure grade A-1 baloney.