How did everyone from different regions agree on the 7 days a week? For example, in the 7th century, the romans, the arabs, the persians all spoke different languages, have different religions, and have different calendars. How did they all agree on the number of days in a week, and if it was monday in the arab peninsula, was it also monday for the romans?
Somebody may choose to answer this question again, but in the meantime, this question has been covered in the FAQ, specifically with /u/yodatsracist's answer to How did so many cultures of the world settle on a 7-day week? Did it have specific advantages over whatever they were using before (like moons, or fortnights, or whatever)? and the ensuing conversation. You may also be interested in the answer to When was it decided which day of the week it was, in order for today to be Wednesday?.
Always more to be said, but this section of our FAQ may be of some interest.