We do not really know as there is very little written about this. What we do know is that the Ancient Greek and Romans used an eight day week which were also divisible by their calendar. So they could use the same calendar every year which made planning of festivals and other events easier. The calendar also followed the lunar cycle. We start finding references to the modern seven day week a few decades before year one. The days were named after the seven known planets at the time who were again named after gods. This is still largely true today. The seven day week became more and more popular over the eight day week. Especially as the calendar were reformed and two extra months were added to complete the year meaning the eight day week did not fit the new calendar. If there ever was a book discussing the change from the eight day week to the seven day week or the reasoning behind the naming of the days after the planets it have been lost to history.
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