They considered the sun and the moon to be "planets" along with Mercury, Venus, Mars, Jupiter, and Saturn. That's the source of the "sacredness" of the number seven, and the reason the week has 7 days.
In Babylonian they were the equivalent of Sunday, Moonday, Marsday, Mercuryday, Jupiterday, Venusday, and Saturnday.
Also note the names that don't correspond well in English (Tuesday to Friday) still do correspond in French: mardi, mercredi, jeudi ("jove" is the short form of "jupiter"), vendredi.
They're different in English just because we substituted Norse gods' names: Tiw, Woden, Thor, and Frigg.