This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
As we saw in the previous section, the planets have given the week
days their names following this order:
Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun
Why this particular order?
One theory goes as follows: If you order the "planets" according to
either their presumed distance from Earth (assuming the Earth to be
the centre of the universe) or their period of revolution around the
Earth, you arrive at this order:
Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn
Now, assign (in reverse order) these planets to the hours of the day:
1=Saturn, 2=Jupiter, 3=Mars, 4=Sun, 5=Venus, 6=Mercury, 7=Moon,
8=Saturn, 9=Jupiter, etc., 23=Jupiter, 24=Mars
The next day will then continue where the old day left off:
1=Sun, 2=Venus, etc., 23=Venus, 24=Mercury
And the next day will go
1=Moon, 2=Saturn, etc.
If you look at the planet assigned to the first hour of each day, you
will note that the planets come in this order:
Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus
This is exactly the order of the associated week days.