The seven-day week is a timekeeping oddity. Unlike days, months, and years, the week doesn’t align with any celestial reality, and it doesn’t divide elegantly into existing periods of time. For example, there aren’t 52 weeks in an average year — there are 52.1428571429. So how did this happen? Babylonians, the ancient superpower of Mesopotamia, put a lot of stock in the number seven thanks to the seven observable celestial bodies in the night sky — the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. This formed the seven-day week, which was adopted by the Jewish people, who were captives of the Babylonians in the sixth century BCE. Eventually, it spread to ancient Greece and elsewhere thanks to the battle-happy Macedonian Alexander the Great. Efforts have been made throughout history to reform the seven-day week, but this oddball unit of time has become ingrained in many religions, including Judaism, Christianity, and Islam, rendering any sort of tweak pretty unlikely.