Planetary hours are an ancient theory that each of the 24*7 hours of the week is "ruled" by a certain celestial object in the solar system. Among other consequences, this theory gave rise to the names of the weekdays in most European languages.
Is it completely arbitrary or could it have some basis in what actually happens in the sky? Like certain planets being more/less prominent in the certain hour of the night.
After all, other astrological concepts are based on what can or cannot be seen in the night sky at a certain time, say, you are a Taurus if the constellation of Taurus could be seen in the sky when you were born.