SE-Ed: Why Does a Week Have Seven Days?

Why does a week have seven days? Why not five, or ten, or even twelve? A day comes from the Earth turning once. A year comes from one trip around the Sun. But the week? That one is a little stranger. Let's start with the Moon. Every night, its shape changes a little. Each big change takes about seven days. Long ago, this made the Moon a handy clock. There's just one problem, though. The numbers don't fit perfectly, so the Moon isn't the whole story. Thousands of years ago, people in Babylon studied the sky. They noticed seven bright lights that moved among the stars. The Sun, the Moon, and five planets they could see. For them, seven became a very special number. The Jewish calendar also has a rest day every seventh day. You can still see these old ideas in English. Sunday is the Sun's day, and Monday is the Moon's. Saturday was named after the planet Saturn. And Thursday comes from Thor, the Norse god of thunder! In French, Tuesday is "mardi", the day of Mars. Believe it or not, the Romans once had eight-day weeks! Slowly, the seven-day week spread across their empire. In the year 321, Emperor Constantine made Sunday a day of rest. Since then, much of the world has followed the same pattern. Some countries tried to change it, though. After the French Revolution, France switched to ten-day weeks. Workers got only one day off in ten! Not surprisingly, people hated it, so Napoleon brought back the old week. In 1929, the Soviet Union tried five-day weeks instead! Family members often had different days off, so they rarely met. By 1940, the seven-day week was back again. So the week isn't really a law of nature. It's a habit we've shared for thousands of years. And honestly, who would want to give up the weekend?