how and when did the whole world end up using a seven day week when there are multiple calendars? It’s not an obvious number…?
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It’s an astrological/astronomical thing, and as others have said, its creation was likely motivated by agricultural needs. The days of the week are each assigned to a heavenly body — one of the seven ones visible with the naked eye. The order of the assignment follows a system of so-called “planetary hours”, in which each hour of each day of the week is assigned to a planet, following a sequence called the “Chaldean order” (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, in that order, which is by apparent distance from the Earth, farthest to closest). Since there are 24 hours per day, 7 elements of the Chaldean order, and the remainder of 24 divided by 7 is 3, this results in the days of the week being sequentially named after every third element of the Chaldean order (i.e. Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus, in that order).
This latter order becomes hilariously obvious to speakers of Romance languages (e.g. French, Italian) and Indic languages (e.g. Hindi, Bengali) once they’re aware of this fact, as the names of the days literally mean “day of X”, where X is one those heavenly bodies, or something that represents it (e.g. for Monday: “Lundi” = “day of Luna/moon” in French, “Somvar” = “day of Som (deity of the moon)” in Hindi). The Germanic languages (e.g. English) loosely follow this, with their days named after Norse deities (e.g. “Tuesday” = “day of Tíw”, who happens to be considered the Norse equivalent of the Roman god Mars, which corresponds to e.g. the French word for Tuesday, “Mardi”).
Astrological and astronomical ideas were very widely shared even in antiquity, which is why this 7-day system with naming of hours and days following the Chaldean order is prevalent and consistent throughout Eurasia. Cultures far outside that region used different systems. For example, in Mayan timekeeping, there is no notion of a week. The closest thing is the 13-day cycle of the Tzolk’in calendar, but that isn’t even consistent, with a 5-day “leap” period occurring after every twenty 13-day cycles to generate a 265-day calendar. Aside from the unnamed 5-day leap period, days in this calendar are assigned both a number between 1 and 13, and a name from a sequence of twenty names — funnily enough, this system is sort of an inverse of the Gregorian calendar, giving a unique name to each day of their 20-day “months”, and numbering the days of their 13-day “week”; in contrast to us _numbering_ the days of our months, and _naming_ the days of our week.
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