week

See also: numbering of weeks in the year

The week is the only unit of the calendar whose length is not an interval determined by the motion of the earth or the moon. It has been suggested that the length of the week reflects the interval between market days.

The seven-day week is established in the Old Testament (Genesis). For many families, it is still the interval between having to go grocery shopping.

Names of the days of the week

In many calendars the days of the week are simply numbered, or have names based on number names. The Jewish (except for the Sabbath) and French Republican calendars are examples. The Quakers in colonial America refused to use day names with pagan roots, and referred to Thursday, for example, as the "fifth day."

Our present day names began in astrology during the Roman empire. Each day was associated with the “planet” that rules its first hour. In those pre-Copernican times, the seven “planets” were (in order of what was thought to be their distance from us) Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon. The first hour of the first day of the week was associated with Saturn, the second hour with Jupiter, and so on, the cycle beginning to repeat with the 8th hour. Since 24 (the number of hours of the day) isn't evenly divisible by 7, the 25th hour–the first hour of the second day–was associated with the sun. The table below, numbering the hours in a week, shows how they cycled through the “planets.” Red numbers indicate the first hour of a day.

Saturn	1	8	15	22	29	36	43	50	57	64	71	78	85	92	99	106	113	120	127	134	141	148	156	163
Jupiter	2	9	16	23	30	37	44	51	58	65	72	79	86	93	100	107	114	121	128	135	142	149	157	164
Mars	3	10	17	24	31	38	45	52	59	66	73	80	87	94	101	108	115	122	129	136	143	150	158	165
sun	4	11	18	25	32	39	46	53	60	67	74	81	88	95	102	109	116	123	130	137	144	151	159	167
Venus	5	12	19	26	33	40	47	54	61	68	75	82	89	96	103	110	117	124	131	138	145	152	160	168
Mercury	6	13	20	27	34	41	48	55	62	69	76	83	90	97	104	111	118	125	132	139	146	153	161
Moon	7	14	21	28	35	42	49	56	63	70	77	84	91	98	105	112	119	126	133	140	147	154	162

This gives the days of the week the order: Saturn, sun, moon, Mars, Mercury, Jupiter, Venus.

Later, under the influence of the Roman religion Mithraism, dies solis replaced dies Saturnis as the first day of the week, which incidentally made the day names more acceptable to Christians, who conducted their religious observances on Sunday and already considered it the first day of the week.

The day names based on planets were officially adopted by the Emperor Constantine in 321 ce.

The original Latin names are evident in the day names in languages derived from Latin (see the modern French names in the table below), but sometime before the 12th century, in some Northern European countries the Roman gods were replaced by Northern European counterparts. In Old English,

Tiw, the god of war, replaced Mars, so that the second day of the week became tīwesdœg.
Odin replaced Mercury, creating wödnesdœg.
Thor, god of thunder, replaced Jupiter, thunresdœg
frigedœg, thought to be from Frigga's day, a translation of Dies Veneris.

Roman astro- logical	Dies Solis (Sun day)	Dies Luna (Moon day)	Dies Martis (Mars' day)	Dies Mercurii (Mercury's day)	Dies (Jupiter's day)	Dies Veneris (Venus' day)	Dies Saturnis (Saturn's day)
Modern French	dimanche	lundi	mardi	mercredi	jeudi	vendredi	samedi
Old English	sunnandœg	mōnandœg	tīwesdœg	wödnesdœg	thunresdœg	frigedœg	sœterndœg
Modern English	Sunday	Monday	Tuesday	Wednesday	Thursday	Friday	Saturday

F. H. Colson.
The Week: An Essay on the Origin and Development of the Seven-day Cycle.
Cambridge: Cambridge University Press, 1926.

S. Gandz.
The origin of the planetary week or the planetary week in Hebrew literature.
Proceedings of the American Academy for Jewish Research, volume 18, pages 213-254. (1949)

Eviatar Zerubavel.
The Seven Day Circle.
New York: The Free Press, 1985.