Day65:Why are there seven days in a week?

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No one’s quite sure how there come to be seven days in a week. Long ago, it may have been the time between one market day and another, or maybe it was just a quarter of a moon month. But after all this time, a seven-day week feels just about right.
market day:
a quarter of :
just about: almost/very nearly, 几乎；近乎；差不多