This is EXACTLY why the Word of Truth must be rightly divided.
There are no special days for the body of Christ,.... BUT for the children of Israel there is a PERPETUAL weekly Sabbath.
Exod 31:12-18 (AKJV/PCE)
(31:12) ¶ And the LORD spake unto Moses, saying, (31:13) Speak 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. (31:14) Ye 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. (31:15) Six 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. (31:16) Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, [for] a perpetual covenant. (31:17) It [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. (31:18) ¶ And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.
Those two things are completely contradictory. (no special days and a perpetual weekly Sabbath).
I'm afraid the contradiction is in your application of the term "for ever" as if it meant "without end."
Exodus 21:5-6 KJV
(5) And if the servant shall plainly say, I love my master, my wife, and my children; I will not go out free:
(6) Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him
for ever.
There's an example of the scope of the word (phrase) in another context. The servant would serve his master "for ever" but he is not still serving his master. "For ever" lasts as long as the applicable context. It doesn't mean without end. Besides, didn't verse 16 (from your citation) say that it was part of a
perpetual covenant? That same perpetual covenant which was broken?
Jeremiah 11:10 KJV
(10) They are turned back to the iniquities of their forefathers, which refused to hear my words; and they went after other gods to serve them:
the house of Israel and the house of Judah have broken my covenant which I made with their fathers.
Zechariah 11:10 KJV
(10) And I took my staff, even Beauty, and cut it asunder,
that I might break my covenant which I had made with all the people.
Zechariah 11:13-14 KJV
(13) And the LORD said unto me, Cast it unto the potter: a goodly price that I was prised at of them. And I took the thirty pieces of silver, and cast them to the potter in the house of the LORD.
(14)
Then I cut asunder mine other staff, even Bands, that I might break the brotherhood between Judah and Israel.
Bands murdered the other party covenant. The temple was destroyed. Israel was scattered. The marriage covenant also ends with "until death do us part." Have you read what was involved in that covenant? It was
conditional. Do you remember what it promised? (It wasn't eternal life.) And that covenant is over ...
Jeremiah 31:31-33 KJV
(31) Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
(32)
Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD:
(33) But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
That new covenant he speaks of is fulfilled in Christ. See Hebrews 10:16. It is not like the old covenant, it is not a handwriting of ordinances.
Hebrews 10:15-17 KJV
(15)
Whereof the Holy Ghost also is a witness to us: for after that he had said before,
(16) This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
(17) And their sins and iniquities will I remember no more.
Galatians 3:7-9 KJV
(7)
Know ye therefore that they which are of faith, the same are the children of Abraham.
(8) And the scripture, foreseeing that God would justify the heathen through faith,
preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
(9) So then they which be of faith are blessed with faithful Abraham.
Galatians 3:16 KJV
(16)
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
I would point out that in the same chapter where Paul emphasizes that "seed" is singular he also uses "gospel" in the singular and says that this is the gospel preached unto Abraham.