The planetary hours are based on an ancient astrological system, the Chaldean order of the planets. This is the sequence: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, then repeating endlessly Saturn, Jupiter, etc.
The planetary hours use the Chaldean order to divide time. Each planetary hour of the planetary day is ruled by a different planet.The planet that rules the first hour of the day is also the ruler of the whole day and gives the day its name.
Each planetary hour is associated with a planet. These can be used to fine-tune a magickal working. This is one step up from performing magick on the day ruled by the planet best suited for the goal. This will give you a specific hour to work the magick.
So say youre planning to do a money spell. Money and prosperity is ruled by Jupiter, so youd want to do it on a Thursday. But to make the spell more precise and give it more "weight", you can calculate the planetary hours for the upcoming Thursday.
Planetary hours are also good for when you cant wait till the appropriate day. If you needed that money, like, yesterday, and today is Saturday, do the spell today but aid it along by doing it during one of the Jupiter hours.