Eviatar Zerubavel's
Seven Day Circle is a detailed and complex history of time telling.
It really helps to understand that in a calendar we're trying to measure three distinct (and variable) cycles, none of which fits precisely into the others, and each of which imposes its own rhythms on human life. Attempts to break each from the current 7/12/365 basis have virtually all failed.
The first is the Earth's rotation about its own axis -- different when measured with respect to the stars or the Sun.
The second is that of the Moon about Earth.
The third is of the Earth about the Sun.
We overlay them on each other and pick rough correspondences.
Much of the division has to do with Babylonian time reckoning, based on 360, and its factors: 2 * 2 * 2 * 3 * 3 * 5. From that you find 2, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 45, 72, 90, 180, and 360. A seven-day week doesn't fit this directly but is close (between 6 and 8), and a 30-day month also fits well.
Then you come to realize that all timekeeping, _especially_ that which picks a specific starting point, is arbitrary.
The book also details several attempts to change the system, particularly following the French and Russian revolutions (oh, and the reason for that being the October revolution -- and there's the question of what was the last nation to adopt the Gregorian calendar, and why the output of 'cal 1752' is what it is ... and why even _that_ is arbitrary (it depends on where you're specifying the date).
http://www.powells.com/biblio/61-9780226981659-2