Today’s Wonder of the Day was inspired by Yoh from , . Yoh Wonders, “Why are there seven days in a week?” Thanks for WONDERing with us, Yoh!

Do you have a favorite day of the week? Maybe it's Monday, because you just can't wait to get back to school after a long weekend. Perhaps it's Wednesday, since that's the night you have piano lessons. Or could it be Friday, Saturday, or Sunday? Many people look forward to the weekend!

Whatever day of the week tends to delight you most, we know it has to be Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. Those are the only choices we have, because a week has only seven days. But have you ever stopped to WONDER why that is?

After all, a typical year has 365 days. Why don't we have 73 five-day weeks? Or 26 14-day weeks? When you think about it, couldn't we divvy up those 365 days anyway we like? Why have we always had seven-day weeks then?

The easy answer is because it's been that way for a really long time. The seven-day week dates back thousands of years. Historians believe it probably got its start with the ancient Babylonians.

Our other time periods can be traced to natural sources, such as the movement of the Earth, Moon, and Sun. For example, Earth's rotation on its axis gives us our day. The movement of Earth around the Sun gives us our year. The Moon's phases even approximate our months. However, there is no natural explanation for the seven-day week.

Some experts believe the ancient Babylonians may have adopted a seven-day week to approximate the individual cycles of the Moon throughout the month. These cycles, however, only imperfectly line up with a seven-day week, requiring an extra day or two to be added to one week every few months.

Why did the ancient Babylonians settle on a seven-day week then? Some experts believe it's because the ancient Babylonians believed the number seven had a mystical significance to it. This belief may have stemmed from their focus on the seven heavenly bodies they knew of at that time: the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn.

The tendency to perform rituals every seven days may have given rise to the seven-day week. Experts also believe these same celestial bodies may have led other ancient cultures, including the Chinese and Japanese, to adopt their own seven-day weeks.

The development of the seven-day week in other cultures can be traced to the creation story in the Bible. According to the Book of Genesis, God created everything in the world in six days and then rested the seventh day. Many believe this provided a model for early cultures to follow: work six days and rest on the seventh day.

Our modern calendars still adhere to the seven-day week. Scholars believe its formal adoption by Roman Emperor Constantine in 321 solidified its acceptance worldwide. Constantine established Sunday, the Christian Sabbath, as the first day of the week and Saturday, the Jewish Sabbath, as the last day of the week.

Many calendars and cultures around the world still observe Sunday as the first day of the week. Practically, however, with the modern five-day work week and two-day weekend, many people informally consider Monday to be the first day of the week and Sunday the last.

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