Originally, the Greeks named the days of the week. They were named after
* the sun
* the moon
and 5 Greek gods or goddesses;
* Ares   (god of war)
* Hermes (messenger of the gods)
* Zeus (king of the gods)
* Aphrodites (goddess of love and beauty)
* Kronos (a Titan, father of Zeus)

The Romans kept the days named after
the sun and
the moon.
They replaced the Greek gods with their own:
* Mars (god of war)
* Mercury (messenger of the gods)
* Jove or Jupiter (king of the gods)
* Venus (goddess of love and beauty)
* Saturn (same as Kronos)

The Germanic or Teutonic people of northern Europe also renamed the days of the week.  They kept the Greek days named after the sun and the moon.  They kept the Roman day named after Saturn. They renamed the other four days of the week after their own gods and goddesses.

Sunday         (sun's day)
Monday        (moon's day)
Tuesday        (Tui's day)   Named after Tui, the Germanic god of war. Also called Twia or Tyr.
Wednesday    (Woden's day)   
                   Woden was the chief of the Teutonic people. In Norse myths, Woden is called Odin.
Thursday      (Thor's day)   Thor was the Norse god of thunder and was the son of Odin.
Friday          (Freya's day)   
                    Frey was sometimes called Fria. She was the Norse goddess of love and beauty.
Saturday       (Saturn's day)  Saturn was the Roman god of farming