Why is Sunday called Sunday? Because it’s the day the pre-Christian Anglo-Saxons and other Germanic peoples set aside for the worship of the sun.
Caesar’s Gallic Wars, published in the first century B.C. There, he described the Germans as caring nothing for the gods of the Romans. He said they worshiped only those deities that they could actually see, the ones that provided something of real, tangible use: the sun, the moon and fire. Caesar said they knew nothing of the other gods, not even by rumor.
The Anglo-Saxons came from Germany. They conquered much of the island of Britain, beginning in the fifth century A.D., were also worshipers of the sun, the moon, and fire. And this is reflected in the Old English name for Sunday, Sunnandæg, meaning “day of the sun.” Old English was the language of the Anglo-Saxons. In Germanic mythology the sun is personified as a goddess variously named Sunna or Sól (Sun in Old Norse).
It is because of a common heritage of worshipping the sun in pagan times that there are equivalents of the English name Sunday in the various other modern Germanic languages: Dutch: zondag; German: Sonntag; Danish and Norwegian: søndag; Icelandic sunnudagur (Sunna’s Day); and Swedish söndag.
During the Christianization of these countries, it was customary to take over existing pagan temples and use them instead for Christian ceremonies. This approach commandeered the holiness of the site and redirected it in the direction of Christian reverence.
In the same way, existing holidays devoted to pagan gods were appropriated for Christian holidays, and the holiest day of the week devoted to the German’s greatest deity, the sun, was designated as the high day for Christian worship. However, it was easier to get people to worship a new god than it was to get them to call the day by a new name. So Sunday remained Sunday, even when (most) people stopped worshipping the sun.
In European countries using Romance languages, however, the name for Sunday is derived from Latin dies Dominica, "the Lord's day." Thus, in French it’s dimanche; in Spanish and Portuguese it’s domingo; in Romanian, duminică; and in Italian, domenica.
Source: Patch.Com