Days of the Week
Astronomy and ancient gods

The week as we know it was introduced by astromancers around the 3rd century, replacing the Roman calendar in use until then. The order of the days was explained by Dio Cassius (and Chaucer gave the same explanation in his Treatise on the Astrolabe). According to Cassius, it was a principle of astrology that the heavenly bodies presided, in succession, over the hours of the day. The Ptolemaic system asserts that the order of the heavenly bodies, from the farthest to the closest to the Earth, is: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. (This order was first established by the Greek Stoics.)

If the first hour of a day is dominated by Saturn (), then the second hour is dominated by Jupiter (), the third by Mars (), and so on, so that the sequence of planets repeats every seven hours. Therefore, the twenty-fifth hour, which is the first hour of the following day, is dominated by the Sun; the forty-ninth hour, which is the first hour of the next day, by the Moon. Thus, if a day is labelled by the planet which dominates its first hour, then Saturn’s day is followed by the Sun’s day, which is followed by the Moon’s day, and so forth, as shown below.

In English, the Anglo-Saxon names of the days of the week obscure this relationship somewhat (see Names).

The same order can be derived “geometrically” from an acute heptagram, the {7/3} star polygon (as 24 Mod 7 = 3). The luminaries are arranged in the same Ptolemaic/Stoic order around the points of the heptagram. Tracing the unicursal line from one planet to the next gives the order of the weekdays.

According to Aleister Crowley, in The Book of Thoth (1944), “It is believed that this neat discovery is due to the late G. H. Frater D.D.C.F.” (Pt. 1, Ch. 1). (Although, as Crowley calls the figure a hexagram, his reliability is somewhat suspect.)

According to some sources this weekday heptagram is considerably older —

It was with the adoption and widespread use of the seven-day week throughout the Hellenistic world of mixed cultures that this heptagram was created. [http://www.symbols.com/encyclopedia/29/2915.html]

Modern ordering

According to the ISO 8601 standard the week begins on a Monday. This corresponds with the term weekend for the Saturday and Sunday. However following Constantine’s decision to make the first day of the week the day of religious observance, Sunday may also be considered the first day of the week in historically Christian countries. In this regard calendars exist in two varieties. The traditional Sunday-first system is used by some English speakers and much of Latin America, while most of continental Europe uses the ISO order.

Names

In English all the days of the week are named after the ruling luminary, with most of the names coming from Anglo-Saxon gods and goddesses. Sunday and Monday are named directly from the sun and moon, although the Anglo-Saxon goddess Sunne is implicit in the name of the sun itself. Saturday is the only day named after a Roman god. Other Germanic languages generally follow the same pattern, but Dutch is the only other that preserves all the astronomical names.

In most Romance languages (Italian, Spanish, French, etc.), the names of the days except Saturday and Sunday come from Roman gods via the Latin. Welsh, the closest living language to that of Roman Britain, faithfully preserves all the Latin names, even though the language itself is not directly descended from Latin.

The early Christian Church was uncomfortable using names based on “false” pagan gods, and introduced a simple numerical nomenclature which persists in many European languages, although in Slavic languages the numbering starts on Monday rather than Sunday (anticipating ISO 8601).

Astronomical

Numerical

Notes

1. The Romance languages (Italian, Spanish, French, etc.) break from the pattern of using the Roman associations in favor of a Christian association: Sunday being the Christian day of rest, these days are named after the “day of our Lord”. This is the norm in languages, such as Portuguese, where names faithfully follow Church usage.
2. The Romance languages, Old High German and German, and the Slavic languages have words for Saturday that reflect the Jewish Sabbath.
3. German, Finnish, and Estonian call Wednesday, prosaically, “mid-week”; Icelandic “mid-week day”; Polish, Russian, etc. “middle”.
4. Old Norse, Swedish (and other North Germanic languages), and Finnish and Estonian (Finnic languages) call Saturday “washday” or “bathday”, as it was the traditional day for washing and bathing.
5. Icelandic sunnudagur and mánudagur are astronomical, persisting presumably because there’s no explicit reference to pagan gods.

Resources

Michael Falk, “Astronomical Names for the Days of the Week”, Journal of the Royal Astronomical Society of Canada, Vol. 93, p.122. 1999JRASC..93..122F. [Abstract] 

Amy X, Days of the week and months of the year in many different languages