Sunday is named after the Sun
The name "Sunday", the day of the Sun, is derived from Hellenistic astrology, where the seven planets, known in English as Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon, each had an hour of the day assigned to them, and the planet which was regent during the first hour of any day of the week gave its name to that day. During the 1st and 2nd century, the week of seven days was introduced into Rome from Egypt, and the Roman names of the planets were given to each successive day.
Germanic peoples seem to have adopted the week as a division of time from the Romans, but they changed the Roman names into those of corresponding Teutonic deities. Hence, the dies Solis became Sunday (German, Sonntag).
The English noun Sunday derived sometime before 1250 from sunedai, which itself developed from Old English (before 700) Sunnandæg (literally meaning "sun's day"), which is cognate to other Germanic languages, including Old Frisian sunnandei, Old Saxon sunnundag, Middle Dutch sonnendach (modern Dutch zondag), Old High German sunnun tag (modern German Sonntag), and Old Norse sunnudagr (Danish and Norwegian søndag, Icelandic sunnudagur and Swedish söndag). The Germanic term is a Germanic interpretation of Latin dies solis ("day of the sun"), which is a translation of the Ancient Greek heméra helíou.
The p-Celtic Welsh language also translates the Latin "day of the sun" as dydd Sul.
In most Indian languages, the word for Sunday is Ravivāra or Adityavāra or its derived forms — vāra meaning day, Aditya and Ravi both being a style (manner of address) for Surya i.e. the Sun and Suryadeva the chief solar deity and one of the Adityas. Ravivāra is first day cited in Jyotisha, which provides logical reason for giving the name of each week day. In the Thai solar calendar of Thailand, the name ("Waan Arthit") is derived from Aditya, and the associated colour is red.
In Russian the word for Sunday is Воскресенье (Voskreseniye) meaning "Resurrection". In other Slavic languages the word means "no work", for example Polish: Niedziela, Ukrainian: Недiля, Belorussian: Нядзеля, Croatian: nedjelja, Serbian and Slovenian: Nedelja, Czech: Neděle, and Bulgarian: Неделя.
The Modern Greek word for Sunday, Greek: Κυριακή, is derived from Greek: Κύριος (Kyrios, Lord) also, due to its liturgical significance as the day commemorating the resurrection of Jesus Christ, i.e. The Lord's Day.
In Korean, Sunday is called 일요일 Il-yo-Il, meaning "day of sun".