In order to address the questions related to the special day of worship being Sunday we must first look at God's opinion in general concerning "special days". It is true that in the Old Testament God had set the Sabbath as being a day of rest, a holy day.

Exodus 20:10, 11

10  But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:

11  For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.

These instructions to keep the Sabbath holy and to keep it as a day of rest were part of the Law that God gave to Moses. This Law also included the 10 Commandments along with numerous other laws. Many do not realize that the Law of Moses included far more than the 10 Commandments.

This Law that God gave to Moses was then given to the Children of Israel in the Old Testament so that they would better be able to walk with God. It is very important to note that the Law was given to Israel. It was the best that God could do in order to help man to walk according to His ways. The Law was also given before Jesus Christ had come and completed his work in order to redeem mankind.

As time went on there were those of Israel who kept the Law (including the Sabbath) and there were those who did not. In the fulness of time God sent Jesus Christ as the man who would redeem His people. By his walk of obedience to God, his death, his resurrection and his ascension to sit on the right hand of God, Jesus Christ redeemed man from the curse of the Law:

Galatians 3:10-13

10  For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them.

11  But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith.

12  And the law is not of faith: but, The man that doeth them shall live in them.

13  Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree:

The Law was a curse for there was no way that any man could have kept it perfectly. Note that the above verses indicate that there was no man that could ever have been justified by the law. Justification could only come by believing in the accomplished work of Jesus Christ.  Jesus Christ redeemed man from being under the Law that God had given.

Romans 10:4

4  For Christ is the end of the law for righteousness to every one that believeth.

Therefore, all those who have made Jesus Christ their Lord and have believed that God has raised him from the dead benefit from all the work that Jesus Christ has done on their behalf - and that includes redemption from being under the Law that had been given to Israel. According to the scriptures we are no longer under the Law.

Galatians 3:24, 25

24  Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.

25  But after that faith is come, we are no longer under a schoolmaster.

If we are no longer under the Law then that means that the adherence to the 10 Commandments (which were part of the Law) and the keeping of the Sabbath (which was part of the Law) no longer applies to those who are Christians. The Law was given to Israel and it applied to Israel. It is not applicable to Christians. Note the following verses from Galatians:

Galatians 4:4-11

4  But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,

5  To redeem them that were under the law, that we might receive the adoption of sons.

6  And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.

7  Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.

8 ¶  Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.

9  But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?

10  Ye observe days, and months, and times, and years.

11  I am afraid of you, lest I have bestowed upon you labour in vain.

There is no longer any special observances of days, months, times or years - and that would have to include the Sabbath. Now clearly if one is a believer in the Lord Jesus Christ it would be foolish to think that he or she would want to break the 10 Commandments just because he or she is not bound by them. Likewise, if a believer in the Lord Jesus Christ chooses to keep one day a week as being special for whatever reason - then they are free to do so.

Romans 13:8-10

8  Owe no man any thing, but to love one another: for he that loveth another hath fulfilled the law.

9  For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.

10  Love worketh no ill to his neighbour: therefore love is the fulfilling of the law.

Note how the above verses indicate that if a believer walks by the love of God, then that will indeed fulfill the Law. Christians are challenged to walk on a much higher plane: to walk by the love of God. By his walk of obedience, Jesus Christ has purchased great freedom for the believer. Let's be sure as followers of the Lord Jesus Christ to not put anyone back under a bondage from which they have been set free.