We all know a year is the length of time it takes for the Earth to orbit the Sun. We know a day is a full rotation of the planet. But what about the week? The seven-day week is the international standard. But where did it come from? Other cultures have observed four-day, five-day, even thirteen-day weeks. In modern times, the French experimented with a ten-day week and the Soviet Union with five-day and six-day weeks. The seven-day week is not written into the stars or seasons or cycles of nature. It’s written into the Bible. It may not have started there. Some scholars believe the Hebrews inherited the seven-day week from the Babylonians. Whatever the case, the seven-day week became fundamental to Jewish life because it was prescribed in the Bible as the standard length of the week. The tradition was continued by Christians in the first century AD. At the beginning of that era, Rome was in the midst of a transition. The Romans had long observed an eight-day market week, meaning that those who lived outside the cities would come to the city markets after seven days of work. Julius Caesar encouraged a seven-day week in his calendar reforms, which took effect in 45 BC. But the eight-day and seven-day weeks existed side-by-side until Emperor Constantine made the seven-day week official in AD 321. And so it was that the seven-day week, spread throughout the Roman Empire and eventually through Christian cultures around the world. So the next time you hear that famous Beatles song, “Eight Days a Week,” you’ll know that perhaps Ringo and the gang were thinking of the Roman market week.