Best Answer: I've heard many things regarding the reasons for Sunday observance of the Sabbath. As many have stated before, Christians merely followed the example of the apostles. The apostles did this to remember Jesus' resurrection. Some have speculated that Jesus' resurrection occurred on a special "eighth day" in which God overcame sin, this special day being the modern 1st day of the week.
I myself am a Seventh-day Adventist. And yes, this denomination has been given a lot of criticism for their "extreme" focus on a Seventh day Sabbath. I can see where these criticisms come from. Despite all of this, I am still a Seventh-day Adventist. I am also "somewhat" tolerant of others preferred days of worship, which would be a shock to many of my fellow Adventist peers. Here's my personal view on Sabbath observance:
Anything we do as Christians should coincide with the Bible and the people that influenced it. The Bible was very clear... to those living during the times it was written. Over time, cultures have changed, and ways of thinking have certainly evolved from what they were in Jesus' time. Because of this change in culture, a tiny bit of ambiguity has inevitably crept into the Scriptures. We, as human beings, are left to interpret our own meanings (hopefully with the guidance of God). This is the reason why Christianity is separated into many denominations. Satan has stealthily manipulated God's children by putting these denominations against each other, having them emphasize their differences in opinion rather than rejoicing in their similarities (mainly the Gospel). As Christians, we should respect other Christian's beliefs, so long as they truly believe that the Bible implicitly supports their beliefs.
So, as a Seventh-day Adventist, if I meet a Sunday worshiper and ask him why he believes that Sunday is the Sabbath, what answer would allow me to respect his beliefs? Scripture and... that's it. On the other hand, if he were to state that the Edict of Constantine says so (a creed which actually stems in bringing pagan worshipers into the Church), or that the Pope says so, or that he doesn't know, I am inclined to state that there may be a flaw in his reasoning. I have to be blunt about this: I don't believe the Pope to be the vicar of Christ. No where in the Bible does God state that He needs a mediator between Him and us. Jesus Himself IS the mediator. The Pope, by no means, is a bad person; don't get me wrong on that.
In conclusion, I'd suggest personally asking a Catholic to explain why he/she worships on Sunday. If he/she gives Scriptural evidence, and Scriptural evidence alone, respect his/her beliefs. If he/she mentions the Pope or anything outside of the Bible, you may have to question their beliefs.
(If you guys are wondering how I can believe that Ellen White was a prophet and that the Pope is not the Vicar of Christ, here's my reasoning: Ellen White's teachings directly coincide with the Scriptures. She never states anything that goes against the Bible. Her teachings merely enhance the Word. The Pope on the other hand, has changed the order of the Ten Commandments and changed the day of worship, both of which directly clash with what the Bible says. The Word may be a little outdated, but that certainly does not make it less reliable. You should believe that Jesus was the Word manifested in flesh (John 1:14), and you should also believe that Jesus is the alpha and the omega (Revelation 1:8). We can then safely conclude that the Word will last from beginning to end, unchanged and undisturbed. It is perfect, and any attempts to change the Bible can and should be seen as a direct act against God.)
Source(s): The Bible... and thoughts (hopefully from God) that have been on my mind.