It's true, the ten Words/Commandments and all of the Mosaic Law were given to Israel specifically, and in Acts 15 we don't find a reference to keeping the Sabbath day when the 'rules' for gentile believers are formulated by the Apostles and Elders in Jerusalem.
All words/commandments (except the Sabbath day command) have been interpreted as a universal code of ethics for all believers throughout Church history AFAIK.
The interpretation of what the Sabbath day command for gentile believers differed quite a bit in time and between various theological schools of thought, e.g.:
- early Jewish believers / contemporary Messianic fellowships / Seventh-Day-Adventists: still observe the Sabbath day instruction and rest on Saturdays
- Catholic Church (from late Early Church period) / Reformed / Puritan: Sunday is the new Sabbath day
- Martin Luther / John Calvin: principles of rest and worship are important, but the specific day less so
Early Church fathers still acknowledged the fundamental difference between the Sabbath day (the 7th day), and Sunday (the 1st day).
Personally I would mention the following:
- the validity of only 9 out of the 10 commandments for gentile believers seems a bit odd; the 10 words are the apex of Gods instruction and 'handwritten' on the two tablets; they somehow seem to be a unified set
- Genesis 2:2-3 tells us God blessed the seventh day and made it holy; this seems a creation-order statement and may therefore apply to all mankind (not just to the Israelites). One could argue Moses and/or Israelite scribes wrote those words so they wrote it with the Israelite context in mind, but that is speculation.
- Isaiah 56:6-7 speaks about foreigners who love the name of YHWH in connection with those who keep the Sabbath day and keep His covenant - this already is a wider application than just Israelites.
- Isaiah 66:22-23 contains a prophecy "From new moon to new moon, and from Sabbath to Sabbath, all flesh shall come to worship before me, declares YHWH" - so in the future the Sabbath still acts as a time marker for mankind, not just the Israelites.
- In Mark 2:27 Yeshua says ''The Sabbath was made for man (=mankind), not man (=mankind) for the Sabbath". The Greek word 'anthrōpos' is used here which represents 'mankind' in general sense. This hints at a wider application than just Israel.
- Hebrews 4:9-10 mentions the principle of 'Sabbath rest' - although the audience is Jewish, the the idea of Sabbath/rest principle is still current.
- nearly ALL Christians I know would acknowledge it's healthy to rest at least one day per week; if that is the case, and recognising the practicality of doing that with a given pattern, why not choose the seventh day as it lines up with Genesis 2:2-3 and the Ten Words?
What to do with all that is personal - I've switched to celebrating Sabbath/resting on the seventh day (Saturday) a couple of years ago, but I still happily also attend a church on Sunday if that fits my agenda/circumstances.