The Planetary Hours are a belief that sections of each day are ruled by certain heavenly bodies, and that these times of day can be utilized by those who understand their secrets to improve success in certain types of ventures.
Folks like Jim Haskins and Tarostar have printed slightly incorrect versions of "Venus Hours" which have been popularly repeated: the claim that 2 AM, 9 AM, 4 PM and 11 PM on a Sunday, 6 AM, 1 PM and 8 PM on Monday, etc, are Venus Hours is wrong. This is a problem that goes back to the 18th century at least -- the Petit Albert itself mentions the mistake. The error stems from the notion that planetary hours align to hours on the clock -- they do not. As the Albert puts it: "In order that there be no mistakes about the hours that each planet rules [...] one needs to reckon the first hour from the sunrise, and not by midnight, as some people have erroneously claimed." In other words, the planetary hours are reckoned by a solar clock. The first 12 hours last from sunrise to sunset and are equally divided into whatever length of time yields 12 "hours" out of that span. These hours will rarely come out to 60 minutes in length, and will change their timespan almost every day as the time of sunrise and sunset changes. The night time hours are figured the same way, from sunset to sunrise, dividing the span into 12 sections. The number of each section is used to figure the hour. So, for example, the first of the 12 daytime hour sections for Sunday, is the Sun hour.
Sunday's hours (daytime; nighttime)
Sun: 1, 8; 3, 10
Moon: 4, 11; 6
Mercury: 3, 10; 5, 12
Mars: 7; 2, 9
Saturn: 5, 12; 7
Venus: 2, 9; 4, 11
Jupiter: 6; 1, 8
Monday's hours (daytime; nighttime)
Sun: 5, 12; 7
Moon: 1, 8; 3, 10
Mercury: 7; 2, 9
Mars: 4, 11; 6
Saturn: 2, 9; 4, 11
Venus: 6; 1, 8
Jupiter: 3, 10; 5, 12
Tuesday's hours (daytime; nighttime)
Sun: 2, 9; 4, 11
Moon: 5, 12; 7
Mercury: 4, 11; 6
Mars: 1, 8; 3, 10
Saturn: 6; 1, 8
Venus: 3, 10; 5, 12
Jupiter: 7; 2, 9
Wednesday's hours (daytime; nighttime)
Sun: 6; 1, 8
Moon: 2, 9; 4, 11
Mercury: 1, 8; 3, 10
Mars: 5, 12; 7
Saturn: 3, 10; 5, 12
Venus: 7; 2, 9
Jupiter: 4, 11; 6
Thursday's hours (daytime; nighttime)
Sun: 3, 10; 5, 12
Moon: 6; 1, 8
Mercury: 5, 12; 7
Mars: 2, 9; 4, 11
Saturn: 7; 2, 9
Venus: 4, 11; 6
Jupiter: 1, 8; 3, 10
Friday's hours (daytime; nighttime)
Sun: 7; 2, 9
Moon: 3, 10; 5, 12
Mercury: 2, 9; 4, 11
Mars: 6; 1, 8
Saturn: 4, 11; 6
Venus: 1, 8; 3, 10
Jupiter: 5, 12; 7
Saturday's hours (daytime; nighttime)
Sun: 4, 11; 6
Moon: 7; 2, 9
Mercury: 6; 1, 8
Mars: 3, 10; 5, 12
Saturn: 1, 8; 3, 10
Venus: 5, 12; 7
Jupiter: 2, 9; 4, 11
To make an example, let's pretend that you have a Saturday where the amount of daylight will be exactly 9 hours from sunrise, and sunrise takes place promptly at 7:30 AM. This would mean the daylight planetary hours will each last for 45 minutes. In that case, 7:30 to 8:15 is Saturn hour (the first hour), 8:15 to 9:00 is Jupiter hour (the second hour), 9:00 to 9:45 is Mars hour (the third hour), 9:45 to 10:30 is Sun hour (the fourth hour), 10:30 to 11:15 is Venus hour (the fifth hour), 11:15 to noon is Mercury hour (the sixth hour), 12:00 to 12:45 is Moon hour (the seventh hour), 12:45 to 1:30 is Saturn hour (the eighth hour), 1:30 to 2:15 is Jupiter hour (the ninth hour), 2:15 to 3:00 is Mars hour (the tenth hour), 3:00 to 3:45 is Sun hour (the eleventh hour) and 3:45 to 4:30 is Venus hour (the twelfth hour). The sun sets at 4:30, and from there the same system is done for reckoning the evening hours; and in this example, each evening planetary hour will last 75 minutes. So, 4:30 to 5:45 will be Mercury hour (the first nighttime hour), 5:45 to 7:00 Moon hour (the second nighttime hour), 7:00 to 8:15 Saturn hour (the third nighttime hour)... and so on.
If you have trouble in remembering or in calculating all these hours, it should be known that the first hour of each day will be the same planet as the weekday. Therefore, casting a spell at sunrise on Monday will be casting it during Moon hour, sunrise on Friday will be Venus hour, and so on.
In a previous posting, I have given some simple advice for utilizing planetary magical influences using incense. The astrological order of the days was explained by Vettius Valens and Dio Cassius (and Chaucer gave the same explanation in his Treatise on the Astrolabe). According to these authors, 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.)
In astrological theory, not only the days of the week, but the hours of the day are dominated by the seven luminaries. 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 with the Sun, Venus, Mercury, and the Moon, 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.
According to Vettius Valens, the first hour of the day began at sunset, which follows Greek and Babylonian convention. He also states that the light and dark halves of the day were presided over by the heavenly bodies of the first hour of each half. This is confirmed by a Pompeian graffito which calls 6 February 60 a Sunday, even though by modern reckoning it is a Wednesday. Thus this graffito used the daylight naming convention of Valens whereas the nighttime naming convention of Valens agrees with the modern astrological reckoning, which names the day after the ruler of the first daylight hour.