If you, like me, read old books on ceremonial magic, astrology, gardening, herbal medicine, or any combination of those subjects, you’ve probably read something like, “do X in the hour of Saturn”. For a lot of modern folks, that has no meaning. So, what in the blazes is the author talking about?
Chaldea was a small nation located in southeastern Babylonia. It flourished from about the 10th century BCE to around the 6th century BCE. The Chaldeans are thought to be the fathers of astrology and their system of planetary influence on Man (and Earth in general) continues today.
I’m not sure I’ve ever met a person who didn’t know his or her Sun sign. They might even know which element rules their sign and perhaps which planet. (I’m a Virgo, ruled by Earth and Mercury.) Even if that’s the extent of your knowledge of astrology (it is mine), you can imagine carrying that further. Rather than pull a Tarot card in the morning to see what their day may be like, some people will do an astrological chart every day. This was so meaningful during the Renaissance that kings – and even Popes – had a professional astrologer on staff, or at least on call. Nostradamus is one of the more famous astrologers of the Renaissance era.
Anyways, back to planetary hours. According to the Chaldeans, not only each segment of the zodiac but each day and even each hour was ruled by a planet. Therefore, if you wanted or needed to do something important, you should do it at the appropriate hour of the appropriate day or night for best results. (To figure out which day is ruled by which planet, the first hour of the day in the chart below shows the day’s ruler.)
Calculating the proper hour isn’t quite as easy as looking at a clock. Yes, it’s based on a 24-hour day (actually, a 12-hour day and a 12-hour night) but each hour isn’t necessarily 60 minutes long. Nor will the hours exactly correspond all over the world. My day (or night) may be a little longer or a little shorter than someone who lives north or south of me.
To calculate your planetary hours: find out when your sunrise and sunset is. The hours between sunrise and sunset are your day and between sunset and the next sunrise, your night. Translate each into total minutes, then divide by twelve to find your “hours”.
For example, here in my neck of the woods, today’s sunrise was 6:50 a.m., sunset will be 7:14 p.m., and tomorrow’s sunrise will be 6:48 a.m. Therefore, my day is 11 hours, 24 minutes long (684 minutes; each hour is 57 minutes long), my night will be 12 hours 34 minutes long (754 minutes; each hour is 62.8 minutes long, or round to 63).
Then you have to consult a chart. 
Then you need to take into account that I’m on Daylight Savings Time (ick) so I have to subtract an hour from my clock time to get to “real” time. So, if it was auspicious for me to do something tonight (on a day ruled by Jupiter) at the hour of Saturn, I’d need to do it in the second hour of the “real” night, which would be between 7:17 p.m. and 8:20 p.m.
Confusing? A little. A major pain? You bet. I understand there are websites that will do your calculations for you. (There’s probably even an app for that.) However, to my way of thinking, any self-respecting witch is going to do their own calculations – it’s all part of the spellwork. I don’t go this far…I prefer short-and-sweet spells and most of ’em are done spur-of-the-moment. But I thought I’d put this out there to help further understanding of old grimoires and such; and because even the Farmer’s Almanac still has zodiacal information for planting and other things.