"...the Chaldean astrologers introduced the 7-day week which has come down into the present.

The number was convenient because the seers recognized seven planets: Saturn, Jupiter, Mars, Sun, Venus, Mercury and the Moon, each of which governed one hour of the day.

If, for illustration, Saturn ruled the first hour of a certain day followed by each of the 'planets' in turn, he also ruled the eighth, fifteenth, and twenty-second hours.

Jupiter was lord of the second, ninth, sixteenth, and twenty-third hours; Mars presided over the third, tenth, seventeenth, and twenty-fourth hours, and the Sun took charge of the first hour of the succeeding day.

Since the planet which ruled the first hour gave his name to the entire day, Sunday thus followed Saturn-day, and this was the way the names of the days of the week came into existence out of ancient Chaldean astrology."

Reference: Makemson