Ladies, gentelmen, please stop this two and fro, there is a very basic answer to sunday worship.
Vincejohn is right, saturday is the holy sabbath, of which we keep holy, this is not changed by sunday worship, however this command has not been done away with but the church reccognises that the new covenant brought with it another holy day, a one no less important but a day that fullfils the revelation of God to all mankind... "The Lords Day"
The lords day is not an abolishment of the Sabbath, it is a day equal to it and a day that pleases the lord of the Sabbath to be celebrated in inclusion to the sabbath or as a fullfilment of the sabbath... did Jesus not say
"The Sabbath was made for man, and not man for the Sabbath."
and wasnt easter sunday created for the salvation of man, not man for the salvation of easter sunday
Sunday is the lords day, the day we celebrate his ressurection, on the sabbath we celebrate the creation of the world, on the sunday we celebrate the salvation of the world, these two days are as pleasing to God as each other, didnt Jesus our saviour who rose from the dead on Sunday (the lords day) his day of victory over sin and death say...
For the Son of Man is Lord of the Sabbath
Constantine didnt Change the sabbath to the sunday for the pagan sun worshipers, the sabbath has never been changed, Jesus said " i have come to fullfill the law, not to abolish it" and it was Jesus Christ, himself that fullfilled the divine revelation of almighty god.... I am the begining and the end, the begining of the week, the lords day and the seventh day, the end of the week, the sabbath.. fullfilled - not abolished.. the begining and the end, alpha and omega... to him be all Glory and honour forever and ever amen!
Ps, sunday worship by the apostles is alluded to in the bible well before constantine, I know many of you are not at grips with the book of revelation but, when John says
I was in the Spirit on the Lord's day
He litteraly means he was celebrating Mass on Sunday
Anyway I hope I have given you a little insight into the understanding of the lords day and how it is a fullfilment of the sabbath, the celebration of creation and its salvation, how it is a visible sign of the true and living God in the world, how these two days as a fullfilment of Gods plan for the world reflects his most holy self as the begining and the end of all things.
God bless