Christopher, at Conservative Perspective has a post where he shows why he thinks that Saturday should be considered the Sabbath Day. While he emphasized that he doesn’t want me to use the "man’s word", in his post he himself relies on man’s word (he calls it "actual and verifiable history"). I will point out to you both Biblical and historical evidence to that proves that Sunday is indeed the true Sabbath Day. In fairness I will only use a small sample of biblical and historical texts from long before the time of Constantine, on whom Christopher seems content to hang the "blame" for changing the Sabbath.
First, a very important piece of scriptural evidence. There are many passages in the Old Testament that prophesy the end of the exclusive covenant between God and the children of Abraham, Isaac and Jacob, to be replaced with a new and everlasting covenant between God and the whole world, Jews and Gentiles alike. Many of them describe the new worship in prophetic terms. In one particular passage it is specifically stated that the old sabbaths would be brought to an end.
"And I will cause all her mirth to cease, her solemnities, her new moons, her sabbaths, and all her festival times. " (Hosea 2:11, Douay Rheims Bible, referred to hereafter as DRB).
For a New Testament passage that fulfills this prophecy, we have Colossians 2:16-17 -
 Let no man therefore judge you in meat or in drink, or in respect of a festival day, or of the new moon, or of the sabbaths,  Which are a shadow of things to come, but the body is of Christ. [DRB]
Here is the same passage in a popular Protestant version:
16 Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. 17 Which are a shadow of things to come; but the body is of Christ.
[New International Version (NIV)]
Here is a passage from Acts that shows the day they came together to "do this (the Eucharistic sacrifice) in memory of me" as he commanded [Luke 22:19; 1st Corinthians 11:24]:
 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.
7 On the first day of the week we came together to break bread. Paul spoke to the people and, because he intended to leave the next day, kept on talking until midnight.
The first say of the week was known also as the eighth day, the 1st day of the New Creation. It was also commonly known as "The Lord's Day", and in the Book of Revelation we see it referred to as that:
"On the Lord’s Day I was in the Spirit, and I heard behind me a loud voice like a trumpet,"
(Revelation 1:10 NIV)
In the Old Testament the Old Covenant was instituted when God gave a covenant through Moses to the Jews. The Old Covenant was abolished at the cross. Below are some more Biblical passages that talk about the superiority of the New Covenant and our freedom under it from the requirements of the Old Covenant.
6 But in fact the ministry Jesus has received is as superior to theirs as the covenant of which he is mediator is superior to the old one, since the new covenant is established on better promises.
7 For if there had been nothing wrong with that first covenant, no place would have been sought for another.
Freedom in Christ - Galations 5:1-6
1 It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery.
2 Mark my words! I, Paul, tell you that if you let yourselves be circumcised, Christ will be of no value to you at all. 3 Again I declare to every man who lets himself be circumcised that he is obligated to obey the whole law. 4 You who are trying to be justified by the law have been alienated from Christ; you have fallen away from grace. 5 For through the Spirit we eagerly await by faith the righteousness for which we hope. 6 For in Christ Jesus neither circumcision nor uncircumcision has any value. The only thing that counts is faith expressing itself through love.
Here are quotations of Early Christians, showing that the tradition of Sunday worship goes back to the time of the apostles and their immediate successors.
90AD DIDACHE: "Christian Assembly on the Lord's Day: 1. But every Lord's day do ye gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, that your sacrifice may be pure. 2. But let no one that is at variance with his fellow come together with you, until they be reconciled, that your sacrifice may not be profaned. 3. For this is that which was spoken by the Lord: In every place and time offer to me a pure sacrifice; for I am a great King, saith the Lord, and my name is wonderful among the nations." (Didache: The Teaching of the Twelve Apostles, Chapter XIV)
100 AD BARNABAS "We keep the eighth day [Sunday] with joyfulness, the day also on which Jesus rose again from the dead" (The Epistle of Barnabas, 100 AD 15:6-8).
100 AD BARNABAS: Moreover God says to the Jews, 'Your new moons and Sabbaths 1 cannot endure.' You see how he says, 'The present Sabbaths are not acceptable to me, but the Sabbath which I have made in which, when I have rested [heaven: Heb 4] from all things, I will make the beginning of the eighth day which is the beginning of another world.' Wherefore we Christians keep the eighth day for joy, on which also Jesus arose from the dead and when he appeared ascen+ded into heaven. (15:8f, The Epistle of Barnabas, 100 AD, Ante-Nicene Fathers , vol. 1, pg. 147) ,-
150AD JUSTIN: "He then speaks of those Gentiles, namely us, who in every place offer sacrifices to Him, i.e., the bread of the Eucharist, and also the cup of the Eucharist, affirming both that we glorify His name, and that you profane [it]. The command of circumcision, again, bidding [them] always circumcise the children on the eighth day, was a type of the true circumcision, by which we are circumcised from deceit and iniquity through Him who rose from the dead on the first day after the Sabbath, [namely through] our Lord Jesus Christ. For the first day after the Sabbath, remaining the first of all the days, is called, however, the eighth, according to the number of all the days of the cycle, and [yet] remains the first.". (Justin, Dialogue 41:4)
How we answer to the question of whether the true Sabbath is Saturday or Sunday depends on how we treat the tradition and authority of the Church Christ founded. Some people believe that they can divorce the authority of scripture from the authority of the Church. That is not only inconsistent (because the Church, by her divinely given authority, chose the canon of scripture, distinguishing divinely inspired books from those that lack divine inspiration), it is also unbiblical. According to scripture, the pillar and foundation of truth is not scripture itself, but the Church:
1 Timothy 3:15 if I am delayed, you will know how people ought to conduct themselves in God’s household, which is the church of the living God, the pillar and foundation of the truth. (NIV)
When Christ came He did not form a publishing house. He did not write a book, and he did not command that books should be written. He founded a Church. The writing of books of scripture was part of the larger general command to teach and make disciples of all nations, to pass on the traditions that He taught his disciples. That passing on of sacred tradition has always been and always will be through two channels of transmission - word of mouth and epistle, i.e. oral and written tradition:
2Thessalonians 2:15 So then, brothers, stand firm and hold to the teachings we passed on to you, whether by word of mouth or by letter. (NIV)
2Thessalonians 2:15 Therefore, brethren, stand fast; and hold the traditions which you have learned, whether by word, or by our epistle.(DRB)
The authority of scripture is the divine authority God gave to the Church. It is incoherent to reject the latter in the name of the former. It is a misuse of the Bible to lump sacred apostolic tradition in with the merely human tradition rightly condemned in scripture (Mark 7:8 and Colossians 2:8). The same Bible that commands us to reject human tradition that denies the word of God tells us that the word of God will be given to us through sacred tradition. The distinction between sacred apostolic tradition which we are commanded to keep and human tradition which we are commanded to reject is a scriptural distinction and absolutely necessary.
The central issue is in the opposite ways Christopher and I treat the teaching authority of the Church Christ founded. He thinks he can disregard the authority of the Church while claiming to believe the Bible. Scripture does not support his error.
Matthew 16:18 And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it. (DRB)
Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven. 18 And I tell you that you are Peter,[b] and on this rock I will build my church, and the gates of Hades[c] will not overcome it. 19 I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” (Matthew 16:17-19 NIV)
18 Then Jesus came to them and said, “All authority in heaven and on earth has been given to me. 19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.” (Matthew 28:18-20 NIV)
And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth.  Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.  Teaching them to observe all things whatsoever I have commanded you: and behold I am with you all days, even to the consummation of the world. (Matthew 28:18-20) DRB)
"He who listens to you listens to me; he who rejects you rejects me; but he who rejects me rejects him who sent me." Luke 10:16 (NIV)