Sunday definitely has the easiest etymology to figure out of all the weekdays. Sun and day. But there's a reason all the Germanic languages have the traditional day of rest named after a celestial body rather than having it mean "day of the Lord" like all the Romance languages (deriving from Latin dominicus) or "no work day" as in Slavic ones. This is a remnant of our Anglo-Saxon heritage: the pagans who conquered England in the fifth century were fierce worshippers of the sun and the moon, which is also why Monday is named after the moon. Now for the actual etymology! In Middle English, Sunday was spelled sunnenday, and in Old English, it was sunnandæg. This is composed of the parts sunne, meaning "sun" and often personifying the deity that is the sun. Sunne comes from Proto-Germanic sunno, from Proto-Indo-European sohwl, still meaning the "sun". Day comes from dæg, from Proto-Germanic dagaz, from PIE deg, "to burn".
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.