Why the weekday planets skip around instead of following their sky order
Our days honour seven classical sky wanderers, the Sun, the Moon and five planets, yet not in their traditional order of distance. Each day's planet sits three places along that old list from the day before, a pattern left over from astrologers assigning a planet to every hour. Plutarch wrote a treatise puzzling over it around AD 100.
Seven was not inevitable. Ancient Egypt counted ten-day weeks, Etruscans and early Romans used eight, and some West African markets still run on four- or five-day cycles. Sargon of Akkad left the earliest known astrological emphasis on a seven-day span, around 2300 BC, in a culture that revered the number because seven bright bodies moved across the sky. Babylonians marked the 7th, 14th, 21st and 28th of each lunar month as inauspicious days with restrictions, and treated the 19th as a day of anger because it fell roughly 49 days, a week of weeks, after the previous month began.
Those Babylonian weeks reset with each new moon. A continuous seven-day cycle running regardless of the Moon first appears in Judaism, by the 6th century BC at the latest, centred on the Sabbath. Christianity inherited it but shifted the focus to Sunday, and Islam holds congregational prayer on Friday. The Romans meanwhile adopted a planetary week of disputed origin, and in AD 321 Constantine made seven days official across the empire, with Sunday a public holiday.
English names swapped in Germanic gods for Roman ones: Tiw for Mars, Woden for Mercury, Thor for Jupiter. Saturn had no counterpart and kept his day. A rival church system simply numbered the days, surviving in the West only in Portuguese, Galician and Icelandic, and Slavic languages still call Wednesday the middle day. Old English wice probably came from a root meaning turn or change, and sennight, seven nights, recalls the old habit of counting nights, as fortnight still does.
The arithmetic is tidy: a week is 604,800 seconds, and a Gregorian year holds 52 weeks plus a day or two. Exactly 20,871 weeks fit into 400 Gregorian years, so 25 September 1626 and 25 September 2026 were both Fridays. In 1973 John Conway devised his Doomsday rule for finding any date's weekday in your head.
Source: Week
