Day of the week
The concept of a week that repeats indefinitely, and upon which the rhythm of human activity is based was first used by the Jews and probably goes back to the time of Moses. As it is independent of any astronomical phenomenon, it has the advantage of simplicity over the day, month, and year, all of which vary in length to some extent in order to synchronize. The Jewish days had no names other than numbers (with our Sunday as the first day of the week). Like the oblique references to the sun and moon in the description of creation in Genesis 1 (where they are described simply as "a greater light" and "a lesser light"), the philosophical implication of the Jewish week was to demythologize the natural world. In ancient times, the names of the sun, moon, and planets were inextricably tied to names of deities. Numbered days and weekly patterns that ignored the cycles of these deities expressed the contrary view that these objects were inanimate.
Much later, and independently, an astrological week of seven days was constructed beginning with Saturn's day (our Saturday). This likely originated in hellenistic Alexandria in the second century BC. Seven days matched the number of planets by the ancient definition: something that wandered against the backdrop of stars (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon). Horoscopes were written based on the planet associated with the hour of the day and the day of the week. Interest in astrology spread this week throughout Europe in the early centuries of the Roman empire. Hence the English day names all come from planet names directly (Sunday, Monday and Saturday) or indirectly (e.g., Friia was the Nordic god of marriage and fertility, so Venus-day became Friday).
In other parts of the world the week spread with the spread of Christianity
and of Islam. In these places day names are generally based on numbers
or more explicit Christian references (e.g. "the Lord's day" for Sunday).
As both systems have seven days, it long ago became impossible to cleanly
distinguish between the importance of each for later use of the week
(e.g., German has two day names related to the Jewish numbering and five
related to planet deities).
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