The Naming of the Days
The Greeks named the days week after the sun, the moon and the five known planets, which were in turn named after the gods Ares, Hermes, Zeus, Aphrodite, and Cronus. The Greeks called the days of the week the Theon hemerai "days of the Gods". The Romans substituted their equivalent gods for the Greek gods, Mars, Mercury, Jove (Jupiter), Venus, and Saturn. (The two pantheons are very similar.) The Germanic peoples generally substituted roughly similar gods for the Roman gods, Tiu (Twia), Woden, Thor, Freya (Fria), but did not substitute Saturn.
Sunday -- Sun's day
Middle English sone(n)day or sun(nen)day
Old English sunnandæg "day of the sun"
Germanic sunnon-dagaz "day of the sun"
Latin dies solis "day of the sun"
Ancient Greek hemera heli(o)u, "day of the sun"
Monday -- Moon's day
Middle English monday or mone(n)day
Old English mon(an)dæg "day of the moon"
Latin dies lunae "day of the moon"
Ancient Greek hemera selenes "day of the moon"
Tuesday -- Tiu's day
Middle English tiwesday or tewesday
Old English tiwesdæg "Tiw's (Tiu's) day"
Latin dies Martis "day of Mars"
Ancient Greek hemera Areos "day of Ares"
Tiu (Twia) is the English/Germanic god of war and the sky. He is identified with the Norse god Tyr.
Mars is the Roman god of war.
Ares is the Greek god of war.
Wednesday -- Woden's day
Middle English wodnesday, wednesday, or wednesdai
Old English wodnesdæg "Woden's day"
Latin dies Mercurii "day of Mercury"
Ancient Greek hemera Hermu "day of Hermes"
Woden is the chief Anglo-Saxon/Teutonic god. Woden is the leader of the Wild Hunt. Woden is from wod "violently insane" + -en "headship". He is identified with the Norse Odin.
Mercury is the Roman god of commerce, travel, theivery, eloquence and science. He is the messenger of the other gods.
Hermes is the Greek god of commerce, invention, cunning, and theft. He is the messenger and herald of the other gods. He serves as patron of travelers and rogues, and as the conductor of the dead to Hades.
Thursday -- Thor's day
Middle English thur(e)sday
Old English thursdæg
Old Norse thorsdagr "Thor's day"
Old English thunresdæg "thunder's day"
Latin dies Jovis "day of Jupiter"
Ancient Greek hemera Dios "day of Zeus".
Thor is the Norse god of thunder. He is represented as riding a chariot drawn by goats and wielding the hammer Miölnir. He is the defender of the Aesir, destined to kill and be killed by the Midgard Serpent.
Jupiter (Jove) is the supreme Roman god and patron of the Roman state. He is noted for creating thunder and lightning.
Zeus is Greek god of the heavens and the supreme Greek god.
Friday -- Freya's day
Middle English fridai
Old English frigedæg "Freya's day"
composed of Frige (genetive singular of Freo) + dæg "day" (most likely)
or composed of Frig "Frigg" + dæg "day" (least likely)
Germanic frije-dagaz "Freya's (or Frigg's) day"
Latin dies Veneris "Venus's day"
Ancient Greek hemera Aphrodites "day of Aphrodite"
Freo is identical with freo, meaning free. It is from the Germanic frijaz meaning "beloved, belonging to the loved ones, not in bondage, free".
Freya (Fria) is the Teutonic goddess of love, beauty, and fecundity (prolific procreation). She is identified with the Norse god Freya. She is leader of the Valkyries and one of the Vanir. She is confused in Germany with Frigg.
Frigg (Frigga) is the Teutonic goddess of clouds, the sky, and conjugal (married) love. She is identified with Frigg, the Norse goddess of love and the heavens and the wife of Odin. She is one of the Aesir. She is confused in Germany with Freya.
Venus is the Roman goddess of love and beauty.
Aphrodite (Cytherea) is the Greek goddess of love and beauty.
Saturday -- Saturn's day
Middle English saterday
Old English sæter(nes)dæg "Saturn's day"
Latin dies Saturni "day of Saturn"
Ancient Greek hemera Khronu "day of Cronus"
Saturn is the Roman and Italic god of agriculture and the consort of Ops. He is believed to have ruled the earth during an age of happiness and virtue.
Cronus (Kronos, Cronos) is the Greek god (Titan) who ruled the universe until dethroned by his son Zeus.