Leolaia,
Actually, the reason why the week has 7 days is based not on the number of planets or other astrological symbols (if so, these are secondary), but on the lunar calendar, in which the month is exactly 29.53 days long from new moon to new moon. Each week corresponds with a phase of the moon, and since there are four phases, each week is 7.3825 in length. Note that there is a remainder of a third of a day for each week; this is why the actual length of the month varies in a lunar calendar between 29 days and 30 days.
Wikipedia says:
So it is not really clear maybe what is the origin.
I do think, that both could be the origin at the same time.
DB