system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there 7 days in a week

A year is the time it takes for the earth to go around the sun. A month is the time it takes for the moon to go around the earth. A day is the time it takes for the earth to rotate once around its axis. But what is the significance of a week and why is this period used and accepted by pretty much every culture on Earth?
gpt
It's cause of one of the earliest civilizations, the babylonians, had 7 days in their week. Eastern cultures (orientals) had 5 and 10 day weeks. Don't know what the native americans had, probably nothing.
The only reason really for having a week-like thing is to regulate days of business and rest periods. This is usually only needed in a city civilization where they have regular markets and celebrations. 
Otherwise people would normally just worry about the moon phases for their crops.