Planetary week sounds astronomical, like something goes around something in certain amount of time. But in fact the planetary week has it's root in astronomy instead.
The names of the days in a week are based on the classical planetary model where the seven known planets or celestial orbs orbit the earth. The "planets" being the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturnus corresponding the days from Sunday till Saturday. And of course the order doesn't seem to make any sense!
As said the days are based on the classical planetary model where the planets (or orbs) are arranged around the earth based on their distance from earth. The order, also called the Chaldean order of planets defines the planets in the order of Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon based on how fast they move across the sky.
Ok, but they still don't match up? Yeah, that would have been too easy. Now each of those planets was said to rule over the day and each hour of the day is also ruled by a planet. The ruler of the hour follows the Chaldean order e.g. the hour ruled by Saturn is followed by the hour ruled by Jupiter and so on in seven hour cycles. The ruler of the day is then defined by which ever planet rules the first hour of the day. So logically when there are 24 hours in a day each day is followed by a day ruled by a planet three places down the Chaldean order.