Show me anywhere in the NT where it says that we are to Keep the "Lord's Day."
I've already given you ACTS 15 where there were ONLY two requirements for the Christians coming into the faith as far as the Old Covenant was concerned. 1. Avoid idolatry 2. Avoid sexual immorality
The Bible records the abrogation of the Old Covenant Mosaic laws in the "vision" of St.Peter Acts 10, in Col. 2:16 and Gal. 4:10-11, (as you have mentioned) as well as in the Council of Jerusalem Acts 15 which took place 20 years after the Apostles had changed the Jewish sabbath to Sunday and had been keeping holy "the first day of the week", "The Lord's Day".
The NT pays special honor to Sunday although the actual word "Sunday" is not there. In 33AD, Christ rose on Sunday, and He appeared to His Apostles on Sunday. He chose the following Sunday to appear to them when St. Thomas was present. Fifty days later, Christ chose Sunday for the bestowal of the Holy Spirit upon His Church. Pentecost Sunday Acts 2. St.Peter, standing with the 11, preached Christ to the multitudes v. 41-42, "So those who received his word were baptized and there were added about 3,000 souls. 42 And they devoted themselves to the Apostles teaching and fellowship to the breaking of bread and prayers."
"The breaking of the bread" was the special way the early Christians referred to making and distrubiting the sacrament containing the Lord's Body. (St.John 6 + St.Matt.26:26 which we've discussed many times).
So "keeping the Lord's Day" had been going on from 33AD, KFC, from Pentecost onwards, the prayers, the eucharistic communion on the first day of the week form the center of Christian worship. So the first Christians themselves observed Sunday from the very beginning. Acts 20:7, "on the first day of the week when we assembled to break bread.."
The thing is that Sunday observances supplemented rather than supplanted Saturday observance of the Sabbath during the first days of the Church. That's due to nearly all the Christians being converts from Judaism.
When the Gentile converts came into the Chruch, communal life between them and the Jewish Christians was difficult. And that's where Romans 14 and Galations comes in.
Paul wrote in Romans 14:1-5
Him that is weak in the faith receive but not to doubtful disputations. For one believes that he may eat all things; another who is weak, eats only herbs. Le not him that eats despise him that eats not; and let not him which eats not judge him that eats for God has received him....one man esteems one day above another; another esteems every day alike. Let every man be fully persuaded in his own mind. He that regards the day regards it to the Lord and he that regards not the day to the Lord he does not regard it. He that eats eats to the Lord for he gives God thanks; and he that eats not, to the Lord he eats not, and gives God thanks...
Basically what Paul is doing in v1-12 (more than the above) is giving us the proper attitude Christians should have toward each other in debatable areas of conduct (things that are not clearly stated to be wrong). God has received both the weaker and stronger believer.
Speaking to the Galatians who were trying to adhere to the Mosaic Law (Old Covenant) he wrote this in 4:9-11: "But now, after that you have known God or rather are known of God, how turn you again to the weak and beggarly elements where you desire again to be in bondage? (OLD LAW) You observe days and months and times and years. I am afraid of you, lest I have bestowed upon you labour in vain."
He's telling them that they are not acting like heirs of God and are being influenced by the Judaizers observing the days and festivals of the Jewish calendar.
Some of the Jewish converts continued to keep those Jewish observances including maintaining their observance of the 7th day Sabbath laws and this is probably the context of St.Paul's admonition in Romans 14:1-5 and Galatians.
This went on for a while but then there was even more difficulty that arose.
The Jewish Christians had a high regard for the ceremonies of the Old Law and it caused them to go so far as to hold, for a time, that the Gentile Christians should be circumcised, insisting that they had to become Jews before they could become Christians. A vigorous controvery ensued and the issue was finally presented to the Apostles and other leaders, and the Council of Jerusalem was convened in 50AD.
Back to your quote of Acts 15,
I've already given you ACTS 15 where there were ONLY two requirements for the Christians coming into the faith as far as the Old Covenant was concerned. 1. Avoid idolatry 2. Avoid sexual immorality
The finally decree from the Council of Jerusalem is listed 3 times in Acts. Fully in vs. 19-20 and 28-29 and partially in vs. 21, 25.
The first and all important point of the decree was the freedom of Gentile Christians from the Old Law. V. 28, the formula, "For it hath seemed good to the Holy Ghost and to us to lay no burden upon you than these necessary things:" states the Apostles conviction that important Church decisons (that would later be called ex cathedra) were assisted by the Holy Ghost, the Third Person of the Blessed Trinity.
Verse 28 is the dogmatic part which infallibly declares that converts are free of the obligation of circumcision and of the Mosaic law but are subject to the Gospel's perennial moral teachngs on matters to do with chastity. These parts are permanent becasue it has to do with a necessary part of God's salvific will it cannot change.
V. 29 is part disciplinary and can at some time later be changed.
"that you abstain (fast--don't eat) from things sacrificed to idols, and from blood, and from things strangled, .."
The second element in the decree was intended to make the communal life of the Judaic and Gentile Christians less difficult and establish a bridge between them. It asks Christians of Gentile background, out of charity towards Jewish Christians, to abstain, that is, to fast, go without eating, from the animals that have been sacrifice to idols, and from blood and from meat of animals killed by strangulation.
So it was the Council of Jerusalem, followed by the destruction of the Temple in 70AD, which ended the priestly sacrificial services of the Jews that caused the line of demarcation between the Jewish and Christian 7th day Sabbath, which we can see was blurred for a time, to be so clearly defined, and universally accepted.