As far as the seven day week goes, that most likely comes from the Assyrian/Babylonian tradition and was borrowed by other Semitic cultures, including the Hebrews. The seven day Hebrew creation epic is probably based on the Babylonian creation epic, Enuma Elish which was recorded on seven tablets. In the Epic of Gilgamesh, the storm lasts for seven days.
Babylonian months started on the new moon – a holy day was celebrated every seven days starting from the new moon. The number of days in the final ‘week’ of each month was adjusted such that the next month began on the new moon.
Why seven? That answer is probably lost to history though probably has to do with numerology and the ‘magick’ significance of the number to the Babylonian and Assyrians. They in turn may have gotten it from the Egyptians; seven is the sacred number of Osiris.
The Egyptians, by the way, had a ten day week, each month consisting of three weeks, so 30 days. The year contained 13 months, the 13th month consisting of only 5 days to make the full year of 365 days. The tradition and name of the “small month” still exists on the Coptic Christian calendar (the ‘month’ of Nisi).
Many cultures (including Native American), do not have such a thing as weeks; just months/moons. The dividing of one full moon to the next into ‘weeks’ is totally arbitrary, or perhaps I should say determining how many days you want your week to comprise of is more or less arbitrary; it really depends on where in the world you are.