Sunday or Saturday?
Why at ALL by non-Jews?
Exodus 31:13
"Say to the Israelites, 'You must observe my Sabbaths.
17 "Don't think that I came to destroy the law or the prophets. I didn't come to destroy, but to fulfill.
18 For most certainly, I tell you, until heaven and earth pass away, not even one smallest letter or one tiny pen stroke shall in any way pass away from the law, until all things are accomplished.
19 Whoever, therefore, shall break one of these least commandments, and teach others to do so, shall be called least in the Kingdom of Heaven; but whoever shall do and teach them shall be called great in the Kingdom of Heaven. (Matthew 5:17-19 KJV)