why does a week have seven days?

this is shrouded in mystery. some claim its origins are in persia (but i couldn't find any documented evidence on the net), some claim it comes from egypt (but what I read about the egyptian calendar system only mentions a calendar of 365 and 1/4 days, leap year, and 12 months), and, of course, some claim that this is because god created the world in six days, and rested on the seventh. some claim that it is an even division of the 27 day cycle of the moon. some say it is because there were seven visible planets known to man around the time of the beginings of the seven-day calendar. no one really knows for sure.

what is the origin of the name of the days of the week?

we'll start with the babylonians. they bestowed the names of their gods upon celestial bodies whose movement they could observe with their unaided eyes.

they named the planets based on the characteristics they shared with the gods. because there just isn't much babylonian mythological information about the planets and the specific names of the gods they represented, we'll jump straight to the romans and how they named the planets in relation to their gods:
+-------+--------------------+---------------------------+
|       |                    | associated god(dess)      |
|planet | characteristics    | title                     |
+-------+--------------------+---------------------------+
|sun    | brightest object   | apollo/sol                |
|       | in the sky         | god of enlightenment      |
+-------+--------------------+---------------------------+
|moon   | next brightest     | diana/luna                |
|       | object. also, the  | goddess of fertility      |
|       | observable         | goddess of childbirth     |
|       | relations to       | female, sister of apollo  |
|       | the menstrual      |                           |
|       | cycle              |                           |
+-------+--------------------+---------------------------+
|mercury| fastest object     | mercury                   |
|       | in the sky         | messenger of the gods     |
+-------+--------------------+---------------------------+
|venus  | the brightest      | venus                     |   
|       | planet             | goddess of beauty         |
+-------+--------------------+---------------------------+
|mars   | the red planet     | mars                      |
|       |                    | god of war                |
+-------+--------------------+---------------------------+
|jupiter| the largest        | king of the gods          |
|       | planet             |                           |
+-------+--------------------+---------------------------+
|saturn | the slowest        | god of time               |
|       | planet             |                           |
+-------+--------------------+---------------------------+
The planetary names for the days aren't as obvious in modern english as they are in modern romance languages (for example, french: lundi (lune, moon), mardi (mars), mercredi (mercure, mercury), jeudi (jupiter), vendredi (venus), samedi (saturne, saturn), dimanche (from latin: "day of god" is the only one that doesn't follow suit). as the seven-day, god/planet-based calendary spread around the world, it was localized. in the case of people who speak english, the mythological origins of our days is anglo-saxon. here is how we get from jupiter to thursday:
ROMAN DAYS:

           latin name     translation
---------------------------------------
monday     dies lunae     moon's day
tuesday    dies Martis    mars' day ------+
wednesday  dies mercurii  mercury's day --|---+
thursday   dies jovis     jupiter's day --|---|---+
friday     dies veneris   venus' day -----|---|---|---+
saturday   dies Saturni   saturn's day    |   |   |   |
sunday     dies solis     sun's day       +---------------> gods of war, Battle
                                          |   |   |   |
ENGLISH DAYS:                             |   |   |   |
                                          |   +-----------> gods of wisdom,
           old english                    |   |   |   |     communications, poetry1
           name           translation     |   |   |   |
----------------------------------------  |   |   +-------> gods of the sky,
monday     monondæg       moon's day      |   |   |   |     thunder
tuesday    tiwesdæg       tiw's day ------+   |   |   |
wednesday  wodnesdæg      woden's day --------+   |   +---> goddesses of
           Thuredæg,                              |   |     beauty, love,
thursday   Thurredæg,     thor's day -------------+   |     marriage, fertility
           Thunredæg                                  |
friday     Frigedæg       frigg's day ----------------+
saturday   Sæterndæg      saturn's day
sunday     sunnandæg      sun's day
in modern english, sunday, monday, and saturday retain their planetary names. it is surmised that saturday kept it's roman roots because there wasn't a god analogous to saturn, god of harvest and agriculture. the roman words for sun and moon were translated directly. the others come from the anglo-saxon mythology.

tuesday comes from tiw(also known as tyr2), the norse god of war, the roman counterpart to mars.

wednesday is named after woden (also known as odin, the norse god of poetry, and wisdom, the roman counterpart to mercury.

thursday is named for thor (both thor and jupiter are gods of thunder).

friday is from frigg's day (frigg was the wife of odin, who respresented beauty, marriage, motherhood, love and fertility, similar to the greek venus).

so why are the days in that order?

well, knowing that the days are named for gods/planets, it would seem to make sense that there is an astronomical or astrological reason for the order.

nope

the babylonians devised the following system (which the romans adopted in 321CE when emperor constantine moved from an 8-day calendar to a seven day calendar after having been converted to christianity-- a wonderful mixture of christian and pagan symbolism):

in their observations of the planets, the babylonians could not determine the distance of the planets, but they could determine how quickly they moved across the sky. from slowest to fastest: they split the day into 24 hours, and each hour was "governed" by one of the planets (each of which represented a god or gooddess). the god which ruled the first hour of the day corresponds to it's name.

for example:

pick a day... Let's say it's saturday. Starting with saturday (saturn), assign the planets from (slowest - fastest) to hours of the day, and cycle through until you've gone through 24 hours:
saturday           sunday              monday
--------------------------------------------------------
saturn   1    +--> sun      1    + -->  moon     1
jupiter  2    |    venus    2    |      saturn   2
mars     3    |    mercury  3    |      ...
sun      4    |    moon     4    |
venus    5    |    saturn   5    |
mercury  6    |    jupiter  6    |
moon     7    |    mars     7    |
saturn   8    |    sun      8    |
jupiter  9    |    venus    9    |
mars     10   |    mercury  10   |
sun      11   |    moon     11   |
venus    12   |    saturn   12   |
mercury  13   |    jupiter  13   |
moon     14   |    mars     14   |
saturn   15   |    sun      15   |
jupiter  16   |    venus    16   |
mars     17   |    mercury  17   |
sun      18   |    moon     18   |
venus    19   |    saturn   19   |
mercury  20   |    jupiter  20   |
moon     21   |    mars     21   |
saturn   22   |    sun      22   |
jupiter  23   |    venus    23   |
mars     24   |    mercury  24   |
              |                  |
continue   ---+    continue   ---+
tomorrow           tomorrow
that's why they are in that order. i think that's pretty neat.



1 there is some discussion as to whether or not mercury and woden are analogous gods. My take on the situation is that they are. Both are considered to be gods of wisdom. woden (also called odin), though typically considered a god of battle and war, made great sacrifices in order to gain wisdom. he sacrificed an eye in order to gain knowledge by drinking from a sacred well guarded by a giant. he also hung himself from Yggdrasil in order to learn the runes (used as an alphabet in some norse countries until 1000CE). the runes as language and as a divination tool correspond well to the idea of mercury as a messenger god.

2 the rune tyr is an arrow pointing upward, similar to the astrological glyph for mars, which is a circle with an arrow pointing upwards and to the right.
   .               --+
  /|\               / \
 / | \           __/
   |            /  \
   |            \__/

  tyr           mars