This is a great question! The easiest answer to your question is simply: substitution! Let's see below.

To start this off lets look at something: The Romans had a 7 day week and it was named after their gods and celestial bodies Dies Solis - Sunday (solis/sol = sun) Dies Lunae - Monday (Luna = moon) Dies Martis - Tuesday (Mars) Dies Mercurii - Wednesday (Mercury) Dies Iovis - Thursday (Iovis = Jupiter, the Romans didn't have a letter J so they used an I) Dies Veneris - Friday (Veneris is Venus) Dies Saturni - Saturday (Saturni is obviously Saturn)

At this time, the sun and the moon were considered planets, so those 2 along with the other planets above represent 7 planets that rotate in the solar system.

First of all, the English language, despite having a vocabulary comprised of 66% Greek/Latin base, is considered a Germanic language, largely because the grammar. What specifics in the English grammar qualify it as a Germanic language I won't go into. Another main and obvious point as to why English is a Germanic language is because....well they are a Germanic people. The word England literally means "Land of the Angles" and the Angles were a Germanic people near the Jutland peninsula. A second wave of Germanic settlement was seen on the island: the Saxons.

The Germanic tribes had their own language and gods detached from the Romans, but it was not immune to influence from the Romans. They simply replaced their words and gods with their own!

Lets start out with Sunday and Monday. This was an easy substitution because Germanic tribes already had a words for sun/moon and day so we can see the substitution applied

Dies Solis - solis = SUN dies = DAY > SUNDAY (the sun's day) Dies Lunae - lunae = MOON and dies = DAY > MOON'S DAY aka MONDAY

The remaining days were a bit tricky. The Germanic tribes didn't worship Mars/Mercury/Jupter, etc...so what did they do? They just substituted with their own gods! *note the Germanic names/spelling below are from old Anglo Saxon (not current German)

Tuesday - named after the Nordic sky god Tiw >TIWSDAEG > TUESDAY Wednesday - named after the god Odin/Wodin = WODNESDAEG > WEDNESDAY Thursday - named after the god THOR = THORDIN/DORDINSDAEG > Thursday Friday - Named after the goddess Frigg/Freyja = FRIGDAEG > Friday

By now, you are realizing something....what about Saturday? You can see above that everything was substituted from the Greco/Roman to the Germanic version. But what about Saturday? Why do we still have Saturday if the Germanic people didn't worship Saturn? It appears that the Germanic tribes didn't have an equivalent substitution of their gods for Saturn, or one that they cared to use, so, they just kept Saturn...Saturday = Saeternesdaeg = Saturn's Day