(Maybe this should be saved for the Friday PC question of the day):
A Day is based on the rotation of the earth
A Month is based on the orbit of the moon
A Year* is based on the orbit of the earth around the sun.
While the Torah defines the construct of a Week - is there any celestial cycle underpinning the concept of a week? IOW - aside from the torah laying out the week of creation and defining shabbos, is there any reason to explain the concept of a week?