Answer: In Christianity we have no days to keep as they did under the law, is the answer to these scriptures.
In the Word of God the days have no name—as Sunday or Monday. We have numbers instead. The seventh day was given to Israel as the Sabbath, the sign or seal of the covenant (Ex. 31:13-1713Speak thou also unto the children of Israel, saying, Verily my sabbaths ye shall keep: for it is a sign between me and you throughout your generations; that ye may know that I am the Lord that doth sanctify you. 14Ye shall keep the sabbath therefore; for it is holy unto you: every one that defileth it shall surely be put to death: for whosoever doeth any work therein, that soul shall be cut off from among his people. 15Six days may work be done; but in the seventh is the sabbath of rest, holy to the Lord: whosoever doeth any work in the sabbath day, he shall surely be put to death. 16Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant. 17It is a sign between me and the children of Israel for ever: for in six days the Lord made heaven and earth, and on the seventh day he rested, and was refreshed. (Exodus 31:13‑17)).
It was never given to Gentiles to observe, except those who settled among the Israelites in the land of Caanan.
When Christianity began, it gave us the first day of the week, because the Lord Jesus Christ rose from the dead on that day; the Holy Spirit came to dwell on that day; the first gospel sermon was preached that day, and as time went on, the Christians came together to break bread on that day (Acts 20:77And 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. (Acts 20:7)), and the Apostle John, while alone in the isle of Patmos, gave to it, by the Holy Spirit’s inspiration, its name when he wrote, “I was in the Spirit on the Lord’s day.”
We have no rules or commandments from the Lord how to observe it. That is left to those who truly love Him, to give Him the first place. The statutes of the country in which we live have made it a holiday, and what better could Christians do than to use it for the One who has loved them and given Himself for them?
Romans 14:1-81Him that is weak in the faith receive ye, but not to doubtful disputations. 2For one believeth that he may eat all things: another, who is weak, eateth herbs. 3Let not him that eateth despise him that eateth not; and let not him which eateth not judge him that eateth: for God hath received him. 4Who art thou that judgest another man's servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand. 5One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind. 6He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks. 7For none of us liveth to himself, and no man dieth to himself. 8For whether we live, we live unto the Lord; and whether we die, we die unto the Lord: whether we live therefore, or die, we are the Lord's. (Romans 14:1‑8) leaves everyone free to please the Lord as he knows best and does not put us under law to observe days. It is a great thing for us to please the Lord and devote our hearts and time to how best to glorify Him.