system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: 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...?
gpt
it takes about 28 days for the moon to go from "full moon" to the next "full moon".

this is something that people allover the world could easily observe (no light polution back then). now you can separate this "moon time" into four periods: 

"kinda full" / "decreasing" / "kinda not there" / "increasing".

each of those 4 phases is now 7 days.