In a vast number of languages, the names given to the seven days of the week (Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday) are derived from the names of the seven heavenly bodies (the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn) which were in turn named after contemporary Hellenistic deities. This system was introduced by the Babylonians and later adopted by the Sumerians. The Roman Empire adopted the system during late antiquity. In some other languages, the days are named after corresponding deities of the regional culture. The seven-day week was adopted in early Christianity from the Hebrew calendar, and gradually replaced the Roman internundinum. Eight-day and seven-day weeks existed side-by-side until Emperor Constantine made the seven-day week official in AD 321; thereafter, the seven-day week spread throughout the Roman Empire and eventually through Christian cultures around the world.
The history of the seven-day week can be traced to ancient civilizations. Sunday remained the first day of the week, being considered the day of the sun god Sol Invictus and the Lord's Day, while the Jewish Sabbath remained the seventh. Most historians agree the seven-day week dates back to Babylonians who started using it about 4,000 years ago. The number 7 was sacred to the Babylonians. Emperor Constantine of the Roman Empire made the Day of the Sun (dies Solis, "Sunday") a legal holiday centuries later.