How do Christians who insist upon keeping the 7th day Sabbath explain Acts 15?

Summary
First, James rejects the Pharisees argument for circumcision, but does require Gentiles, who are living outside Israel, to observe four conditions the Mosaic Law requires of Gentiles living in Israel. He also states Gentiles will hear Moses in the synagogue on the Sabbath. In other words, he does not add the Sabbath: it is a given they will continue the practice.

Second, Christians are to be imitators of God. The command to observe the Sabbath is a particular way in which that is done: work six days and rest on the seventh.

Third, Christians are to be imitators of Christ who observed the Sabbath.

Fourth, the command to observe the Sabbath is given before the Mosaic Law. As such it is similar to the blessing promised to Abraham. It is not altered by the Law.

Fifth, the Sabbath is a sign and an eternal covenant.

Sabbath Proscriptions
Since the Gospels each record incidents where Jesus was accused of violating the prohibition of work on the Sabbath, it is easy to overlook the question of obeying the proscribed actions:

“Six days shall work be done, but on the seventh day is a Sabbath of solemn rest, a holy convocation. You shall do no work. It is a Sabbath to the LORD in all your dwelling places. (Leviticus 23:3 ESV)

Four of the seven Gospel disputes over the Sabbath also report Jesus went to the synagogue [cf. Matthew 12:1-14, Mark 2:23-3:6, Luke 6:1-11, 13:10-17]. Two show Jesus went to the Temple [John 5:1-14; 8:59-9:16]. Only Luke 14:1-6 records a dispute which makes no mention of the synagogue or Temple. However, that event was at a meal.

While it is true Jesus disputed with the Pharisees over their interpretation of what was prohibited, there is no evidence Jesus disputed over what was proscribed. To the contrary, the Gospels show Jesus did what was required: He attended a Sabbath meeting. Given this, it is reasonable to conclude Luke's Sabbath meal took place after the synagogue service.

Furthermore, the Gospels record additional events which occurred on the Sabbath during which there was no dispute over working [cf. Mark 1:21-27, 6:1-2, Luke 4:16-30, 4:31-36]. These all took place in the synagogue. This is further evidence Jesus did what was required on the Sabbath, as He must if He was without sin. Nor is there anything to indicate Jesus did away with the requirement for a holy convocation. There is no reason to believe the disciples who observed the requirement with Jesus while He was alive, would stop after His death and resurrection.

Acts repeatedly states the disciples were in the Temple and Paul is repeatedly in the synagogue on the Sabbath (Acts 13:14, 44; 16:13; 17:2; 18:4). Repeatedly it states Gentiles were present to hear what was said. This is prima facia evidence the requirement to assemble on the Sabbath was being followed. There is no evidence anywhere in the New Testament that Jesus, his disciples, Paul, or the first Christians failed to meet on the Sabbath.

The Scriptural Basis of the Sabbath
Since the requirement to limit work to six days is repeatedly stated in the Mosaic Law, it is easy to overlook the basis for observing the Sabbath came before the Mosaic Law.

The first specific Sabbath observances took place before the Israelites arrived at Mount Sinai. One month after leaving Egypt, the people complained about a lack of food. In response the LORD provided manna. The manna fell for six days, but not on the seventh. That day was a holy Sabbath to the LORD:

²² On the sixth day they gathered twice as much bread, two omers each. And when all the leaders of the congregation came and told Moses, ²³ he said to them, “This is what the LORD has commanded: ‘Tomorrow is a day of solemn rest, a holy Sabbath to the LORD; bake what you will bake and boil what you will boil, and all that is left over lay aside to be kept till the morning.’” ²⁴ So they laid it aside till the morning, as Moses commanded them, and it did not stink, and there were no worms in it. ²⁵ Moses said, “Eat it today, for today is a Sabbath to the LORD; today you will not find it in the field. ²⁶ Six days you shall gather it, but on the seventh day, which is a Sabbath, there will be none.”
(Exodus 16)

Moreover, failing to observe the Sabbath was considered a refusal to keep the LORD's commandments and laws, not the Mosaic Law:

²⁷ On the seventh day some of the people went out to gather, but they found none. ²⁸ And the LORD said to Moses, “How long will you refuse to keep my commandments and my laws? ²⁹ See! The LORD has given you the Sabbath; therefore on the sixth day he gives you bread for two days. Remain each of you in his place; let no one go out of his place on the seventh day.” ³⁰ So the people rested on the seventh day. (Exodus 16)

The Mosaic Law to remember the Sabbath is referring to what the LORD had already put in place before giving the Law to Moses. The Mosaic Law adds to observing the Sabbath:

⁸ “Remember the Sabbath day, to keep it holy. ⁹ Six days you shall labor, and do all your work, ¹⁰ but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates. ¹¹ For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy. (Exodus 20)

The Mosaic Law adds two things to the Sabbath. First, what was solely the responsibility of the individual now has the added responsibility for those with authority. For example, before the Mosaic Law if a slave attempted to collect manna or worked on the Sabbath, the slave violated the Sabbath. After the Mosaic Law, a slave owner also violated the Sabbath if their slave worked, even if the owner did not work. The second addition is a reason for the Sabbath: creation. God's people are to imitate the pattern God used to create.

Paul's letters to the Corinthians and the Ephesians give instruction to imitate Christ and God:

Be imitators of me, as I am of Christ. (1 Corinthians 11:1)
Therefore be imitators of God, as beloved children. (Ephesians 5:1)

God worked six days and rested on the seventh. Observing the Sabbath is a particular may to imitate God. Paul observed the Sabbath; Christ observed the Sabbath. The instructions are clear: the Church is to observe the Sabbath.

A Sign and a Covenant
The Mosaic Law added to the LORD's commandment and it explained the Sabbath is a sign of sanctification and an eternal covenant:

¹² And the LORD said to Moses, ¹³ “You are to speak to the people of Israel and say, ‘Above all you shall keep my Sabbaths, for this is a sign between me and you throughout your generations, that you may know that I, the LORD, sanctify you. ¹⁴ You shall keep the Sabbath, because it is holy for you. Everyone who profanes it shall be put to death. Whoever does any work on it, that soul shall be cut off from among his people. ¹⁵ Six days shall work be done, but the seventh day is a Sabbath of solemn rest, holy to the LORD. Whoever does any work on the Sabbath day shall be put to death. ¹⁶ Therefore the people of Israel shall keep the Sabbath, observing the Sabbath throughout their generations, as a covenant forever. ¹⁷ It is a sign forever between me and the people of Israel that in six days the LORD made heaven and earth, and on the seventh day he rested and was refreshed.’” (Exodus 31)

The Antioch Dispute
The context of the Jerusalem Council begins in Antioch:

¹ But some men came down from Judea and were teaching the brothers, “Unless you are circumcised according to the custom of Moses, you cannot be saved.” ² And after Paul and Barnabas had no small dissension and debate with them, Paul and Barnabas and some of the others were appointed to go up to Jerusalem to the apostles and the elders about this question. (Acts 15)

Gentiles who became Christians, were told they needed to be circumcised. Paul, Barnabas, and others disputed this. As is clear in Galatians, Paul vigorously refuted the need for Gentiles to be circumcised. However, with respect to the Sabbath, this dispute says nothing about whether the Centile Christians were already observing the Sabbath. If they were imitating Christ and God, then it goes without saying they were to continue the practice of meeting on the Sabbath.

In fact James implies the Gentiles were already observing the Sabbath:

For from ancient generations Moses has had in every city those who proclaim him, for he is read every Sabbath in the synagogues.” (Acts 15:21)

Given the importance of the Sabbath, the Pharisees would not ignore those actions. Rather, this statement indicates James fully expected the Gentiles would continue the practice of meeting on the Sabbath. There was no added instruction to continue this practice, because none was needed.

Question:

How do those who insist that the 7th day Sabbath must be kept explain the complete absence of this command in a letter sent to the Gentile church for the express purpose of resolving the issue of which commands these believers should keep?

One might better ask:

How do those who insist that murder is a sin explain the complete absence of this command in a letter sent to the Gentile church for the express purpose of resolving the issue of which commands these believers should keep?

Similarly one could ask about theft, adultery, etc.

The key to understanding this decision of the Jerusalem Council is to realize that this is not a complete list of the commandments that Gentile Christians must keep.
Instead, it is a partial list of the Noahide laws that Gentiles must keep in order to interact with the Jewish community.

I've answered similar questions before.

I'm not going to repeat it all here, but the main points are:

• Christians are not bound by the terms of the Old Covenant with Israel, nor with the laws specific to the levitical priesthood.
• Acts 15 is specific to new and potential Gentile converts.
• Public libraries didn't exist, so converts had to visit synagogues to read and learn, so needed to be accepted by the Jewish community.
• Good Gentiles should obey the 7 Noahide Laws (still a belief of modern Judaism).
• The converts weren't expected to understand and practice all God's laws immediately.
  (In fact, in modern Judaism, people are required to break the sabbath until they have been fully converted (a very long process).)
• To show their sincerity, converts were expected to at least adhere to the 7 Noahide Laws.
• Four of these Laws were common to almost all religions (blasphemy, murder, theft, injustice), so weren't explicitly mentioned.
• Three of the Laws weren't so obvious, so were explicitly mentioned:
  • worshiping idols,
  • sexual immorality,
  • eating meat that had not bled to death (e.g. strangled or diseased, or still alive (oysters)).

It is those three, non-obvious, Noahide Laws that were immediately imposed on new converts in Acts 15.

As the converts learned the truth, and became more comfortable with what was expected of them, they were expected to eventually obey all of God's Laws (excluding those that were specifically given as part of Israel's civil government, or part of the religious rituals for the Levites).

— Do Matthew 5:17-20 and Acts 15 contradict each other? - Biblical Hermeneutics Stack Exchange

For a more detailed explanation, see my answer to:

• Were there implicit laws not referenced in the Acts 15 letter to gentile believers?

Related explanations:

• Is eating blood a sin according to Acts 15:20? - Biblical Hermeneutics Stack Exchange

• Did the Jerusalem council allow believers to eat e.g., rabbit meat? - Biblical Hermeneutics Stack Exchange

I like this question.

You need to look at the logic of James and Peter, not only their conclusion.

1. The broader conversation here is actually about whether Gentiles can enter the Kingdom. Peter says that Gentiles were given the Spirit just like the Jews were.

2. In verse 10 Peter points out that the Jews were sinners just like the Gentiles.

3. The law is not a means of salvation. That doesn't mean it doesn't have information about healthy living. It teaches us to worship God and love people, especially, not murdering and being monogamous.

4. James hits the nail on the head as far as the heart: don't make it difficult for people to connect with God

5. James makes his conclusion based on the idea that Moses has already been preached in these places. Basically people know what the law says. It's between them and God whether they practice holy living. It is not true everywhere in the world that people have heard Moses preached, and this part of James' conclusion isn't applicable to the entire Church therefore.

The value of the Sabbath has been proven scientifically, both as far as preserving workers' health, and as far as increasing the productivity of workers. This pertains to healthy living, and does not contradict any of the 5 main points I stated above. And as point 5 states, people can make up their own mind on that before God. People that practice the Sabbath will be physically and emotionally healthier than those that don't, just like those that worship Jesus, don't commit adultery, honor their father and mother, and don't steal.

Let's not make a big thing about the law in church though, creating division. We can encourage people to read the Old Testament without making it the focus. Those that want to learn more can find guidance from people with more knowledge than them, in addition to reading the Bible themselves.

Jesus, love, the Cross. These are the big deals. Faith hope and love. These 3 things remain, but the greatest of these is love.

The Mosaic laws are separate from God's laws. How? Mosaic laws were social laws but God's laws are universal, having originally been written in stone tablets. 12 of them. The sabbath law is from God, not Moses.

1. Worship only the living God.
2. Do not make false gods
3. Do not serve or worship false gods
4. Do not take the Lord's name in vain
5. Keep the sabbath
6. Honor your father and mother for long life
7. Do not kill
8. Do not commit adultery
9. Do not steal
10. Do not give false testimony against anybody
11. Do not covet your neighbour's house.
12. Do not covet your neighbour's wife, servants or hips possessions

Mosaic laws number in their hundreds.

Jesus said in Matthew 11:29-30 (KJV) 29 Take my yoke upon you, and learn of me; for I am meek and lowly in heart: and ye shall find rest unto your souls. 30 For my yoke is easy, and my burden is light.

Leoni the commandments is important as signified by Revelation 14:12 (KJV) Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.

We have faith in the testimony of Jesus but prove our love towards God by keeping his commandments.

This we can say wholeheartedly that the sabbath law is important for us to be recognised as worthy of the salvation Christ will bring at his return. Break this one law is the same as breaking any of the others; penalty of 2nd death.

The Seventh Day Adventist groups do keep a 7th day Sabbath (and not a Sunday Lord's Day as is mainstream Christian practice.) I am not one of them, but met one of their foremost leaders in Edinburgh when I wandered into an Exhibition for Christian Heritage Year in 1984. He and his entourage came in and he started asking me what I thought. My comments led him to give me, free of charge, two of his books. The first was published in 1977, "From Sabbath to Sunday", the second was published in 1980, "Divine Rest for Human Restlessness". The author? Samuele Bacchiocchi. It wasn't until later that I appreciated how privileged I had been, receiving his generous gifts.

Yet that still does not qualify me to answer this question as if on behalf of Seventh Day Adventists. From what I have gleaned, this is how Dr. Bacchiocchi's understands Acts chapter 15 to "explain the complete absence of this [Sabbath-day observance] command in a letter sent to the Gentile church". Here are extracts that I think are relevant to the question. In a chapter entitled "Jerusalem and the Origin of Sunday" he writes:

"In the year A.D. 49-50 the leaders of the Christian Church met in Jerusalem to deliberate on the basic requirements to be fulfilled by Gentiles who accepted the Christian faith... [Quotes Acts 15:1 and works his way through various verses]

The decision of the council, which is mentioned three times (Acts 15:20, 29; 21:25) with slight variations, provides some insight into the attitude of the Jerusalem Church toward Jewish law. Several points are noteworthy.

The exemption from the circumcision was granted only "to the brethren who are of the Gentiles" (v. 23). No innovation occurred for the Jewish Christians, who still circumcised their children... Moreover, the very provisions proposed by James and adopted by the Council indicate that the Gentiles were not granted indiscriminate freedom from the law. Of the four precepts of the decree, in fact, one is moral (abstention from "unchastity") and three were ceremonial (abstention "from pollution of idols and from what is strangled and from blood" - v. 20)...

This excessive [!] concern of James and of the Apostles (Acts 15:22) to respect Jewish scruples regarding food and association with the Gentiles, hardly allows us to imagine that a weightier matter such as Sabbath observance had been unanimously abrogated.

But how can some interpret the silence of the Council on the Sabbath question as "the most eloquent proof that the observance of Sunday had been recognized by the entire apostolic Church and had been adopted by the Pauline Churches?" [that quote from W. Rordorf, Sunday, p. 219] That such a drastic change in the day of worship had been unanimously accomplished and accepted, without provoking dissension, is hard to believe in view of several factors. The prevailing attitude of the Jerusalem Church, as we have already noticed, was characterized by intransigent respect and observance of Jewish customs and institutions. In such a climate it was practically impossible to change the date of a millenarian institution like the Sabbath which was still highly respected.

The statement which James made to support his proposal is also significant in this regard: "for from early generations Moses has had in every city those who preach him, for he is read every Sabbath in the synagogues" (Acts 15:21)... Some take it as meaning that Jewish Christians need not fear that Gentile freedom would undermine the observance of the Mosaic laws, others understand the verse as meaning that since the precepts of the law of Moses were diligently taught every Sabbath, Gentile Christians must be careful not to offend the prejudices of their Jewish fellow-believers. Still others interpret it as meaning that the Gentiles would certainly not find the prohibitions arbitrary or harsh since they were well acquainted with the Levitical regulations from their habitual attendance at the synagogue on the Sabbath. F.F. Bruce thinks that James' "observation was perhaps intended to calm the apprehensions of the Pharisaic party in the Jerusalem Church, in whose eyes it was specially important that the whole Torah should be taught among the Gentiles."

...they all recognize that both that in its proposal and in its justification James reaffirms the binding nature of the Mosaic law which was customarily preached and read every Sabbath in the synagogues... exclude categorically the hypothesis that the Sabbath had already been replaced by Sunday." From Sabbath to Sunday, pp. 145-148, Samuele Bacchiocchi, The Pontifical Gregorian University Press, Rome, 1977 (Imprimatur, Romae, die 16 Iunii 1975) [bold emphasis mine]

In his other book, Dr. Bacchiocchi includes in its Appendix "A Synopsis of the Published Dissertation" (in From Sabbath to Sunday, just quoted from above). It is twenty pages long, so I will only quote its brief 'Conclusion':

"The conclusion that emerges from our investigation is that the adoption of Sunday observance in place of the Sabbath occurred, not in the Jerusalem Church by apostolic authority to commemorate Christ's resurrection, but rather in the Church of Rome during the early part of the second century, solicited by external circumstances. An interplay of political, social, pagan-religious and Christian factors

• similar somewhat to those which gave rise to the December 25 observance of Christ's birth - made it expedient to adopt Sunday as a new day of worship. The fact that Sunday observance rests on questionable expediency rather than on a Biblical mandate makes it very difficult for religious leaders to articulate compelling theological reasons which are indispensable to promote the proper observance of God's Holy Day." Divine Rest for Human Restlessness, p. 250, Samuele Bacchiocchi, fourth printing Tesar Printing Company, June 1983

Those are a few of author Bacchiocchi's reasons for taking the opposite view of the OPs, regarding the omission of all mention of Sabbath-day-keeping in Acts 15.