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.
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. ... 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.
Sunday is the day of the week between Saturday and Monday. For some Christians, Sunday is observed as a day for worship of God and rest, due to the belief that it is Lord's Day, the day of Christ's resurrection. Sunday is a day of rest in most Western countries, part of 'the weekend'.