Domingo comes from Latin, Dominicus meaning 'of the Lord', from dominus 'lord', 'master'. It's a reference to the Christian god, not the sun.
The first recorded day names referring to the planets comes from Mesopotamia, long before the Hebrews got their act together. In fact, according to Wikipedia, "the name Hebrew probably comes from the word 'habiru,' a term used in 2nd-millennium BCE texts throughout the Fertile Crescent for people variously described as rebels, outlaws, raiders, mercenaries, bowmen, servants, slaves, and laborers." That's how the wandering tribes of shepherds and goatherds would have appeared to the urbane city-dwellers who lived beside them.