Planetary hours have been mostly seen as a restriction in modern magic rather than a tool which can focus and strengthen a ritual.
It is said that you should not do a ritual if you are going to end up doing it on the wrong day. The best time to do a Saturn ritual would be the hour of Saturn on Saturday.
However this is a simplistic and inflexible approach.
What you need to work out is what your ritual is really doing and what planetary powers are involved. So for example you might what to do a binding spell to prevent someone taking a certain course of action. Logically Saturday would be a great day for a binding spell. But would we want to actually use Saturn to do the job? If you look at binding spells within the Greek Magical Paprii Saturn is not used in this role. That is usually left to Seth-Typhon to create a more “active” and aggressive binding. So if you did a working in the hour of Mars on Saturn’s day you would have a strong formula for a traditional binding spell. Effectively you are making Typhon manifest within the sphere of Saturn.
What if you wanted to invoke to cure yourself from smoking? Tobacco is ruled by Mars (set again) but the habit is lunar. So you wold invoke Set on Monday.
If you wanted to write a book? Well the question there is what sort of book? If you were writing a romance you would invoke Venus (romance) in the hour of Venus on Mercury’s day (Wednesday). A Book on magic would be done on the hour of Mercury, a psychology book on the hour of the moon.
A love spell would make some interesting hours decisions. If you want to have sex with a person, you would be done in the hour of Venus on Tuesday (mars) because you are focused on the act of sex itself (which is mars). If you want your love to grow you would use the hour of Jupiter (expansion) on Friday. If you want to meet some you would use the hour of mars in Venus (if you were a woman or gay guy wanting a bloke). If you were wanting a girlfriend you would use the hour of the moon, or Venus on a Friday as these planets represent a woman.
Planetary hours give us the ability to tune our rituals to planetary force to help it become more specific.
They can be helpful in emergencies too. If you need to do a ritual which has to be done straight away, you can use the appropriate planetary hour of the day to try and bring about a result. This is not desirable, and might give a strange result but it is better than nothing. Say you hear on Friday that your best friend has had a heart attack and is in hospital. Waiting until Tuesday (mars surgery, or Sun healing) is not an option. To help the operation you do a mars ritual on the hour of mars on the Friday. The odd result you might expect is that the operation is successful because the person turns out to have a surprising amount of natural regenerative life force, or the surgeon has a sudden blinding flash of creativity.
Failing to use planetary hours also might be one of the reasons why rituals fail or we get odd effects. For example we do a ritual for Money using Jupiter (Thursday). If we do it during the hour of the moon the money will be taken away the moment we make it. If we do it in the hour of Mercury it will be gained either by trickery, gambling or writing… through Venus we might get a loan from our partner. Through Saturn it would be hard work etc.
This means that we need to meditate carefully when we are doing the ritual and what its effects might be using the planetary hours as a clue.
Planetary rulership of Days of the week
Monday is ruled by the Moon.
Tuesday is ruled by Mars.
Wednesday is ruled by Mercury.
Thursday is ruled by Jupiter.
Friday is ruled by Venus.
Saturday is ruled by Saturn.
Sunday ruled by the Sun. ”
Planetary hours take a little calculating. Fortunately there is software and you can find help online. If you want to work it out by hand, take the hour of sunrise and the hour of sunset and divide the time by 12 and then look on the following chart
Planetary Hours of the Day
The same method applies to the hours of the night. Take the hour of sunset and the hour of sunrise and divide the time by twelve.
Planetary Hours of the Night