The biggest “sign” in the sky (short of the sun) is the moon.
Half-moons, full-moon, and no-moon (“new”) happen about every 7 days.
Some “wiseguy” decided that 7 days seemed like a reasonable interval to allow some “rather intellectual” people to rest from “normal” work and do “spiritual” stuff. Some of that “stuff” was to explain his “spiritual intellection” to his family (people).
As it is said, “The sabbath was created for man, not man for the sabbath.” (Mark 2:27)
The point of the sabbath is as a sign of the fact that God created His creation as an orderly thing, which because of it’s orderliness is more a “good home” for us humans, which it wouldn’t be if it were more “random”.
The “four times” per moon cycle (which makes a “cross” by the way) when it’s glaringly obvious that the world is “sensibly ordered” are natural times when nature veritably DRAWS out of us an appreciation of it’s Creator’s great gift!
At some point someone decided that the actual moon-phase was less important than just having a completely predictable 7-day cycle, and shifted this “celebration” into a stable mechanism.
That’s my rationalization, and opinion, and worth absolutely nothing to anyone who thinks it’s worth nothing to them!