Sunday is derived from Old English Sunnandæg “sun’s day”, which came from Old Saxon sunnundag.
Germans changed that but little, using Sonntag
The Greek word, Κυριακή, is derived from Κύριος (Kyrios, Lord), thus The Lord’s Day, which has influenced variations on other European languages, Italian, domenica, French, dimanche, Romanian duminică, and Spanish and Portuguese, domingo.

Thus endeth the lesson.