The Daily Planet
The following is from Daniel J. Boorstin's book The Discoverers: A History of Man's Search to Know His World and Himself.
By the third century the seven-day week had become common in private life throughout the Roman Empire. Each day was dedicated to one of the seven planets. Those seven, according to the current astronomy, included the sun and the moon, but not the earth. The order in which planets governed the days of the week was: sun, moon, Mars, Mercury, Jupiter, Venus, and Saturn. This order was not that of their then supposed distance from the earth, which was the "normal" order in which Dante, for example, later described the zones in the heavens, and in which the names of the planets were recited in the schools down to the time of Copernicus.
Our familiar order of the names for the days of the week came from this order of the planets that the Romans thought "governed" the first hour of each day in turn. The astrologers of the day did make use of the "order" of the planets according to their supposed distance from the earth, to calculate the "influence" of each planet on worldly affairs. They believed that each planet would govern an hour, then in the next hour would give way to the influence of the next planet nearer the earth, and so on through the cycle of all seven planets. After each cycle of seven hours, the planetary influences would begin all over again in the same order. The "governing" planet for each day, then, was the planet that happened to preside over the first hour of that day, and each day of the week thus took its name from the planet that governed its first hour. The result of this way of calculating was to name the days of the week in their now familiar order.
The days of our week remain a living witness to the early powers of astrology. We easily forget that our days of the week really are named after the "planets" as they were known in Rome two thousand years ago. The days of the week in European languages are still designated by the planets' names. The survival is even more obvious in languages other than English. Here, with the dominant planet, are some examples.
[He lists the names of the days of the week in French, Italian, and Spanish, but I'll just show French.]
Sunday (Sun): dimanche
Monday (Moon): lundi
Tuesday (Mars): mardi
Wednesday (Mercury): mercredi
Thursday (Jupiter): jeudi
Friday (Venus): vendredi
Saturday (Saturn): samedi