Why does a week have seven days and how are they ordered?
A week is an artificial unit of time. It has seven days because there were seven geocentric planets. It is however not at all obvious how the names of week days are arranged. The prescription is rather involved.
First, arrange the planets in order of increasing orbital period, which is qualitatively the same thing as increasing distance from the Earth: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.( This is the same list as that of naked eye heliocentric planets with Sun in place of the Earth.)
A week has 24 days. Assign each hour to a planet in succession, but in reverse order. Name the day after the presiding planetary deity of the first hour.
The first hour of the first day belongs to the Moon; it is therefore named Monday. The second hour belongs to Saturn, the third to Jupiter, the fourth to the Mars, and so on. The 8th hour will again belong to the Moon and so will the 15th and the 22nd. The 23rd hour goes to Saturn, and the 24th to Jupiter. The next hour belongs to Mars. Since this is the beginning of a new day, the day is named Tuesday. The 22nd hour of this day will again belong to Mars, the 23rd to the Sun, and 24th to Venus. The next day begins with the hour dedicated to Mercury and is therefore named after it.
The whole procedure can be summed up as follows. Draw acircle and mark seven planets on it order. Take any one. That day is named after it. Skip two names and come to the third. That planet lends its name to the next day.