[Slice of Life]

A moon cycle is about 29.5 days. You want to divide it somehow into smaller units, (rounded to a day). So the question is why not 3 x 10 days or 5 x 6 days. Which moon phases can be easily identified? Full moon, half moon, new moon. half moon, full moon, ... So you divide the lunar month into 4 quarters.

But while the seven days week seems to have been popular, other "weeks" were also in use in some cultures.