According to Acts 20:7, which day was the first day of the week? According to Acts 20:7,

And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight. (KJV)
Ἐν δὲ τῇ μιᾷ τῶν σαββάτων συνηγμένων τῶν μαθητῶν τοῦ κλάσαι ἄρτον ὁ Παῦλος διελέγετο αὐτοῖς μέλλων ἐξιέναι τῇ ἐπαύριον παρέτεινέν τε τὸν λόγον μέχρι μεσονυκτίου

early Christians used to assemble on the first day of the week. But, was this assembly on Saturday night, since Hebrew calendrical days were reckoned from sunset to sunset? Or could the Gregorian calendar have impacted the way this verse was translated since it was the standard calendrical system when the King James Version was published?
The Sabbath is the Lord's day, set apart each week for rest and worship. In Old Testament times, God's covenant people observed the Sabbath on the seventh day of the week because God rested on the seventh day when He had created the earth. After the Resurrection of Jesus Christ, which occurred on the first day of the week, the Lord's disciples began observing the Sabbath on the first day of the week, Sunday (see Acts 20:7).


  original link to this info with more scriptural references to the sabbath day in christianity particularly in the LDS tradition [found on lds.org]


this is what I was taught and as someone who has been both Jewish and Christian this makes the most sense to me as an argument that holds water. Hope that helps!