Origins of the names of the days

The names of the days are in some cases derived from Teutonic deities or, such as in Romance languages, from Roman deities. The early Romans, around the first century, used Saturday as the first day of the week. As the worshipping of the Sun increased, the Sun's day (Sunday) advanced from position of the second day to the first day of the week (and Saturday became the seventh day).

Sunday

The name comes from the Latin dies solis, meaning "sun's day," the name of a pagan Roman holiday. It is also called Dominica (Latin), the Day of God. The Romance languages, languages derived from the ancient Latin language (such as French, Spanish, and Italian), retain the root. Old English Sunnandæg.

French: dimanche; Italian: domenica; Spanish: domingo
German: Sonntag; Dutch: zondag. [both "sun day"]

Monday

The name comes from Old English Mōnandæg, "the moon's day". This second day was sacred to the goddess of the moon.

French: lundi; Italian: lunedi. Spanish: lunes. [from late Latin lunae dies, the day of Luna, "Moon."]
German: Montag; Dutch: maandag. [both "moon day"]

Tuesday

This day was named after the Norse god Tyr, from Old English Tīwesdæg. The Romans named this day after their war-god Mars: dies Marti.

French: mardi; Italian: martedi; Spanish: martes.
The Germans call Dienstag (meaning "Assembly Day"), in The Netherlands it is known as dinsdag, in Danmark as tirsdag and in Sweden tisdag.

Wednesday

The day named to honor Wodan (Odin), from Old English Wōdnesdæg.
The Romans called it Mercurii dies, after their god Mercury.

French: mercredi; Italian: mercoledi; Spanish: miércoles.
German: Mittwoch; Dutch: woensdag.

Thursday

Old English Thu(n)resdæg. The day named after the Norse god Thor; in the Norse languages it is called Torsdag.
The Romans named this day Jovis dies ("Jove's Day"), after Jove or Jupiter, their most important god.

French: jeudi; Italian: giovedi; Spanish: jueves.
German: Donnerstag; Dutch: donderdag (donner/donder, "thunder").

Friday

Old English Frīgedæg, the day in honor of the Norse goddess Frigg.

To the Romans this day was sacred to the goddess Venus, and was known as Veneris dies.

French: vendredi; Italian: venerdi; Spanish: viernes.
German: Freitag ; Dutch: vrijdag.

Saturday

This day was called Saturni dies, "Saturn's Day," by the ancient Romans in honor of Saturn. Old English Sætern(es)dæg.

French: samedi; Italian: sabato; Spanish: sábádo.
German: Samstag; Dutch: zaterdag.
Swedish: Lördag; and in Danish and Norse: Lørdag ("washing day").