A day is a period of time that has existed based on the most obvious natural clock we have: the Sun.  For most places on the Earth, a day is the amount of time that elapses between when the Sun is at one point at the sky (say, the zenith) and the next time the Sun is at that point.  Us moderns define a day as a period of 84000 seconds, or 24 hours exactly, since the amount of time that elapses between the Sun’s position at two successive intervals varies from place to place and according to the seasons.  We like accuracy and uniformity of units, after all, and that can be forgiven, especially when we have trade and travel that crosses massive distances when time relative to one point isn’t the same as time relative to another.

Older systems of reckoning, however, had to go along with nature and her irregularities, and so marked a complete day from two relative times; for instance, Jews and Muslims still do this, defining a day as the time that elapses from sunset to sunset.  Astrologers and occultists did a similar thing, except they measured the day from dawn to dawn.  Occultists, who like to ascribe everything correspondences to the planets and other powers that be, gave each of the seven days of the week a relationship to one of the seven traditional planets.  This relationship is ancient and spans countries and cultures, and is still evident in our own day names: Sunday is ruled by the Sun, and Monday by the Moon.  Romance languages show the relationship even more: French vendredi, “Friday” comes from Latin dies Veneris, “day of Venus”.  The English days of the week were named after the old Norse gods, who were likened to the planetary deities by the Romans.  The complete correspondence is below:

1. Sunday — the Sun (from Old English Sunnandæg)
2. Monday — the Moon (from Old English Mōndæg, but cf. Spanish lunes from Luna)
3. Tuesday — Mars (by way of the Norse Tiw or Tyr, thus Old English Tiwesdæg, but cf. Spanish martes from Mars)
4. Wednesday — Mercury (by way of the Norse Woden or Oden, thus Old English Wodnesdæg, but cf. Spanish miércoles from Mercurius)
5. Thursday — Jupiter (by way of the Norse Thor, thus Old English Þurresdæg, but cf. Spanish jueves from Jove)
6. Friday — Venus (by way of the Norse Frigg or Freya, thus Old English Frigedæg, but cf. Spanish viernes from Venus)
7. Saturday — Saturn (from Old English Sæterdæg ← Sæternesdæg)

Note that not all languages keep to this planetary system, with some languages (like Arabic, modern Greek, and modern Chinese) simply numbering the days, e.g. Sunday being “first day” and Saturday being “seventh day”.  In many Romance languages, Saturday was renamed after the Jewish Shabbat (e.g. Spanish sabado), and Sunday was renamed to something along the lines of “day of the Lord” (e.g. Spanish domingo).  However, in places where the Western astrological system was known or spread, this naming convention of naming the seven days of the week after the seven planets has been surprisingly durable throughout the centuries.  For comparisons of weekday names in different languages, check out this page on Omniglot.

Each day was then seen to be good for a certain kind of activity which related to its ruling planet: Mercury was good for business transactions and travel, Friday for parties and romance, Monday for paperwork and tidying up the home, and so forth.  The occultists who devised this system, fond of complexity as they are, went one step further and ascribed each of the 24 hours of a day a ruling planet as well, so you could mix the powers of different planets together for a complex project or use the amplified power of a planet in its own day and hour for a specific task.

The traditional set of 24 hours weren’t hours as we think of them, however, where each hour is 60 minutes.  They were divisions of daylight and darkness, the diurnal and nocturnal hours respectively, and they changed lengths over the course of the year as nights and days became longer or shorter in turn with the seasons.  The length of one diurnal hour was one-twelfth of the time from sunrise to sunset, and the length of one nocturnal hour was one-twelfth of the time from sunset to the next sunrise.  This was commonsense to people before clocks, of course, since the shadow cast by the Sun would move across the equally-spaced markings on a sundial.  This also meant that a day wasn’t officially over until the next sunrise, so although we might consider 4:00 a.m. on a Tuesday morning to be, well, Tuesday, to the older folks it was still Monday.

Once the lengths of the diurnal and nocturnal hours was known, occultists had to decide how to ascribe the planets to each of the hours.   Since it was fitting that the first hour of the day should set the tone for the coming day, they ascribed the first hour the planet that also ruled that day.  So, the first hour of Saturday was ruled by Saturn, the first hour of Sunday by the Sun, and so forth.  After that, they used the order of the planets that organized them from slowest to fastest, or furthest from the sphere of the Earth to the closest: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and the Moon.  So, if the first hour of Sunday was ruled by the Sun, the second hour would be ruled by Venus, the third by Mercury, the fourth by the Moon, the fifth by Saturn (at this point the order of planets repeats), the sixth by Jupiter, the seventh by Mars, the eighth by the Sun again, and so on.

The weekday heptagram may help illustrate this:.  The order of the planets from slowest to fastest, or from furthest to closest to the sphere of Earth, can be read clockwise starting at Saturn (in the upper right) to the Moon (at the top); this is the order of the planetary hours as well, and all you have to do is just keep going around the circle clockwise.  The order of the rulers of the days of the week are given by the seven-pointed star, going clockwise from the Sun (at the bottom left) to the Moon to Mars and so on.  Just like the days of the week repeat week after week, the star itself goes on and repeats after each iteration.  

To give a full example of each planetary hour rulership of each hour of each day of the week, consult the following table.  Remember that the diurnal hours are those that last from sunrise to sunset on a given day, and the nocturnal hours are those that last from sunset to the following sunrise.  Note how the rulership of each consecutive hour (first hour of Sunday, second hour of Sunday, third hour of Sunday, etc.) follows the planetary heptagram diagram above around the circle clockwise, while the rulership of the same hour on consecutive days (first hour of Sunday, first hour of Monday, first hour of Tuesday, etc.) follows the star of the planetary heptagram.  Also note how the last nocturnal hour of each weekday is the preceding planet in the speed/distance order of the first diurnal hour of the following weekday.

Using this system of planetary days and hours, you can pick convenient times to work on or create a planetary talisman or summon a particular spirit.  In fact, a number of the old grimoires and texts demand that you conjure an angel of a planet only in that planet’s day and hour.  Work related to that planet will go smoother and can help sidestep, though not completely, the problem of finding good elections in astrology for beginning or finishing some task.

As an example, consider July 4, 2011 in Washington, D.C.  It’s a Monday, and sunrise in that city on that day is at 5:47 a.m. local time, sunset at 8:36 p.m., and the sunrise on July 5 at 5:48 p.m.  The time between sunrise and sunset is 14 hours 48 minutes, which yields a diurnal hour of about 1 hour 14 minutes.  The time between sunset and the following sunrise is 9 hours 11 minutes, which yields a nocturnal hour of about 45 minutes.  Based on this, we can calculate the starting times of the planetary hours for all of Monday, July 4, 2011:

Note that the system of calculating planetary days and hours can only work if there is both a sunrise and sunset in the same 24-hour period.  In places of extreme latitude during the summer and winter, when a day (when the Sun is above the horizon) or night (when the Sun is below the horizon) lasts longer than 24 hours, this system breaks down and other systems of magical timing need to be used instead.

There’s more to the hours than just the planets, too: they’re ascribed individual names, angels that watch over them, angels related to the planets, and other information that comes up in some forms of ritual magic.  Thankfully, we don’t have to calculate all this by hand anymore.  The National Council for Geocosmic Research has a handy web application that can tell you the planetary hours for the current day based on your location, and this planetary hour calculator can tell you the planetary hours for any given date and location.  If you have a smartphone, Hours may be of interest and use to you if you’re planning things astrologically on the go.  Further, although not attributed to classical or historical magical literature, this system can be made even more specific for particular durations of planetary minutes and seconds; for that, check out Chris Warnock’s page over on his Renaissance Astrology website.